Tests for PR 8991

[tyrannosaurus-becks]

This PR is only the tests used for verifying #8991. They've been separated out so that PR will only contain the minimum code change necessary for the fix.

One of the factors in AWS renewals being broken was that current tests did not flag issues with renewals. This PR adds the ability to mock the AWS clients used, allowing tests to be easily run in all environments. The approach taken was as close as possible to AWS' recommended approach. There is also precedent for mocking the clients, as seen in the Azure secrets plugin.

The tests added would have flagged the issue. When run on master, they fail with:

--- FAIL: TestEC2LoginRenewDefaultSettings (0.00s)
    use_cases_test.go:134: unable to fetch instance ID from metadata during renewal
=== RUN   TestIAMLoginRenewDefaultSettings
--- FAIL: TestIAMLoginRenewDefaultSettings (0.00s)
    use_cases_test.go:270: unable to retrieve canonical ARN from metadata during renewal

On this branch, they pass.

Additional Tests

• SCP'd this binary onto an AWS EC2 instance (Ubuntu 18.04 LTS in us-east-1) and performed login and renewal as described here for EC2 auth
• Also followed same instructions for logging in and renewing with the IAM method

[jasonodonnell]

LGTM!