Lock calls around Test-ModuleManifest, which is not thread safe

[bergmeister]

PR Summary

This is an alternative, better implementation, compared to PR #1257
Basically the issues below show that Test-ModuleManifest exhibits a concurreny problem in all version of PowerShell and it is not clear yet if that is an issue of the cmdlet itself or something deep within SMA. But as a matter of fact, the only way to avoid it at the moment, is not avoid concurrent calls to Test-ModuleManifest.
Please use a whitespace-ignoring diff due to the indentation shift.

Fixes #901
Fixes #902

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• [x ] 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.

[rjmholt]

This looks good.

You may also find that the PSModuleInfo object itself isn't threadsafe, but I'm not sure.