add test and fix delete default route functionality

[megan07]

Fixes hashicorp/terraform-provider-google#7164

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: fixed bug where `delete_default_routes_on_create=true` was not actually deleting the default routes on create.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 39 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 2 files changed, 39 insertions(+), 2 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=144413"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccIapTunnelInstanceIamBindingGenerated|TestAccIapTunnelInstanceIamMemberGenerated|TestAccIapTunnelInstanceIamBindingGenerated_withCondition|TestAccIapTunnelInstanceIamPolicyGenerated|TestAccIapTunnelInstanceIamMemberGenerated_withCondition|TestAccIapWebBackendServiceIamMemberGenerated|TestAccIapWebBackendServiceIamBindingGenerated|TestAccIapTunnelInstanceIamPolicyGenerated_withCondition|TestAccIapWebBackendServiceIamBindingGenerated_withCondition|TestAccIapWebBackendServiceIamMemberGenerated_withCondition|TestAccIapWebBackendServiceIamPolicyGenerated|TestAccIapWebBackendServiceIamPolicyGenerated_withCondition|TestAccProviderBasePath_setBasePath|TestAccComposerEnvironment_private|TestAccComposerEnvironment_update|TestAccComposerEnvironment_withDatabaseConfig|TestAccComposerEnvironment_withWebServerConfig|TestAccComputeDisk_VSSWindows|TestAccComputeHttpHealthCheck_update|TestAccInstanceGroupManager_targetSizeZero|TestAccInstanceGroupManager_basic|TestAccInstanceGroupManager_versions|TestAccInstanceGroupManager_update|TestAccInstanceGroupManager_autoHealingPolicies|TestAccInstanceGroupManager_stateful|TestAccComputeNetwork_networkDeleteDefaultRoute|TestAccRegionInstanceGroupManager_targetSizeZero|TestAccRegionInstanceGroupManager_versions|TestAccRegionInstanceGroupManager_distributionPolicy|TestAccRegionInstanceGroupManager_basic|TestAccRegionInstanceGroupManager_update|TestAccRegionInstanceGroupManager_autoHealingPolicies|TestAccRegionInstanceGroupManager_stateful|TestAccDataprocCluster_withInternalIpOnlyTrue|TestAccDNSManagedZone_dnsManagedZonePrivatePeeringExample|TestAccDNSManagedZone_dnsManagedZonePrivateExample|TestAccDNSManagedZone_dnsManagedZoneServiceDirectoryExample|TestAccDNSManagedZone_privateUpdate|TestAccDNSManagedZone_privateForwardingUpdate|TestAccDNSPolicy_dnsPolicyBasicExample|TestAccDNSManagedZone_reverseLookup|TestAccDNSPolicy_update|TestAccDNSRecordSet_basic|TestAccDNSRecordSet_modify|TestAccDNSRecordSet_changeType|TestAccDNSRecordSet_ns|TestAccDNSRecordSet_nestedNS|TestAccDNSRecordSet_quotedTXT|TestAccDNSRecordSet_uppercaseMX|TestAccSecurityScannerScanConfig_scanConfigUpdate|TestAccProject_deleteDefaultNetwork|TestAccMemcacheInstance_update|TestAccMemcacheInstance_memcacheInstanceBasicExample|TestAccNetworkManagementConnectivityTest_networkManagementConnectivityTestAddressesExample|TestAccNetworkManagementConnectivityTest_networkManagementConnectivityTestInstancesExample|TestAccNetworkManagementConnectivityTest_update|TestAccOSConfigGuestPolicies_osConfigGuestPoliciesBasicExample|TestAccOSConfigPatchDeployment_osConfigPatchDeploymentInstanceExample|TestAccRedisInstance_redisInstancePrivateServiceExample|TestAccSecurityScannerScanConfig_scanConfigBasicExample|TestAccServiceNetworkingConnection_create|TestAccServiceNetworkingConnection_update|TestAccSqlDatabaseInstance_withPrivateNetwork You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=144415"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 39 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 2 files changed, 39 insertions(+), 2 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=144417"