Test that ec2 vpn client association works with multiple subnets

[bill-rich]

This originally included a fix for a problem that was occurring on the AWS side. They have since fixed the issue, so I have removed the changes that were required to work around the problem. The test should still be included to confirm the issue is resolved, and is not broken again in the future.

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Closes #11586

Output from acceptance testing:

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccAwsEc2ClientVpnNetworkAssociation_ -timeout 180m
=== RUN   TestAccAwsEc2ClientVpnNetworkAssociation_multipleSubnets
=== PAUSE TestAccAwsEc2ClientVpnNetworkAssociation_multipleSubnets
=== CONT  TestAccAwsEc2ClientVpnNetworkAssociation_multipleSubnets
--- PASS: TestAccAwsEc2ClientVpnNetworkAssociation_multipleSubnets (815.08s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	815.118s

[bflad]

Otherwise looks good 🚀

Output from acceptance testing in AWS Commercial:

--- PASS: TestAccAwsEc2ClientVpn_serial (0.98s)
    --- PASS: TestAccAwsEc2ClientVpn_serial/NetworkAssociation_securityGroups (682.09s)
    --- PASS: TestAccAwsEc2ClientVpn_serial/NetworkAssociation_basic (732.78s)
    --- PASS: TestAccAwsEc2ClientVpn_serial/NetworkAssociation_disappears (740.80s)
    --- PASS: TestAccAwsEc2ClientVpn_serial/NetworkAssociation_multipleSubnets (955.03s)

Tests fail (as expected) in AWS GovCloud (US) as the functionality is not supported in that partition.

[ghost]

This has been released in version 3.33.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!