refactor: Move appgw and virtual_network_gateway examples to tests directory

[sebastianczech]

Description

Move appgw and virtual_network_gateway examples into tests folder.

I propose to leave main_test.go as they are currently (validate test, plan test, apply test and idempotence test).
Similarly tests from https://github.com/PaloAltoNetworks/terraform-aws-swfw-modules/tree/main/tests are not included into any GitHub workflow (it's still needed to be developed), so they are not actively used. When we prepare common GitHub workflows for modules integrations tests (not only for AWS, but also for Azure), then we can start extending tests for modules.

Motivation and Context

#41

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.

[acelebanski]

Optionally we could change the name of the appgw test to application_gateway, what do you think? Other than that, I think it's good! 👍

[sebastianczech]

Optionally we could change the name of the appgw test to application_gateway, what do you think? Other than that, I think it's good! 👍

Do you propose to change name of the folder in tests or name of the module (folder in modules) ? Now for both of them we have the same name - can we continue that approach ?

[acelebanski]

Optionally we could change the name of the appgw test to application_gateway, what do you think? Other than that, I think it's good! 👍

Do you propose to change name of the folder in tests or name of the module (folder in modules) ? Now for both of them we have the same name - can we continue that approach ?

Oh, okay I see that logic. I'm wondering why we decided to go with shortcuts for things like appgw, natgw, vnet etc. but not with virtual_network_gateway for instance. But nevermind. :)