Import test refactor for customer gateway

[ryndaniels]

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Relates #8944

Release note for CHANGELOG:

NONE

Output from acceptance testing:

$ make testacc TESTARGS="-run=TestAccAWSCustomerGateway_"     
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -count 1 -parallel 20 -run=TestAccAWSCustomerGateway_ -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSCustomerGateway_basic
=== PAUSE TestAccAWSCustomerGateway_basic
=== RUN   TestAccAWSCustomerGateway_similarAlreadyExists
=== PAUSE TestAccAWSCustomerGateway_similarAlreadyExists
=== RUN   TestAccAWSCustomerGateway_disappears
=== PAUSE TestAccAWSCustomerGateway_disappears
=== CONT  TestAccAWSCustomerGateway_basic
=== CONT  TestAccAWSCustomerGateway_disappears
=== CONT  TestAccAWSCustomerGateway_similarAlreadyExists
--- PASS: TestAccAWSCustomerGateway_disappears (34.17s)
--- PASS: TestAccAWSCustomerGateway_similarAlreadyExists (47.71s)
--- PASS: TestAccAWSCustomerGateway_basic (90.33s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       91.786s

[bflad]

LGTM

--- PASS: TestAccAWSCustomerGateway_disappears (16.83s)
--- PASS: TestAccAWSCustomerGatewayDataSource_ID (19.15s)
--- PASS: TestAccAWSCustomerGatewayDataSource_Filter (19.64s)
--- PASS: TestAccAWSCustomerGateway_similarAlreadyExists (21.73s)
--- PASS: TestAccAWSCustomerGateway_basic (39.31s)

[ghost]

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