Retry Test-Module manifest calls to avoid sporadic failures due to concurrency/thread safety issue in PowerShell itself

[bergmeister]

PR Summary

Fixes #901
Fixes #902

Retry Test-ModuleManifest that can sometimes throw due to a thread safety issue in PowerShell itself (I can repro in versions 5-7 and maybe later I want to look into fixing it in PS itself).
This makes PSSA more stable by retrying the operation and resolves the issue as I manually tested. I am keeping the existing method that it calls public to not break the public API although I don't think anyone is using it.
I tried to write a test for it but although I can reproduce locally, it is hard to write test that reproduces this race condition.
I also opened a PR in PowerShell itself to get it fixed in v7: PowerShell/PowerShell#9860

UPDATE: I opened PR #1258 which might be a better alternative by locking the calls of Test-ModuleManifest

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[bergmeister]

UPDATE: I opened PR #1258 which might be a better alternative by having a lock around the calls of Test-ModuleManifest, therefore closing this in favour of it