Compatibility tests: Use correct test cases object and remove invalid test case

[bergmeister]

PR Summary

The $genericVerCases test case object was not being used due to what seems to have been a copy-paste error.
One test case actually did not work because having a version string with not dot would throw here

I could add the following negative test for it but was wondering why we cannot make it accept single character versions?

        It "Does not parse version string '7' due to missing dot" {
            { [Microsoft.PowerShell.CrossCompatibility.PowerShellVersion] '7' } | Should -Throw
        }

In the case of PowerShell Core we wouldn't even need our own implementation and just use System.Management.Automation.SemanticVersion, which can actually parse versions without a dot.

PR Checklist

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• 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
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[rjmholt]

I could add the following negative test for it but was wondering why we cannot make it accept single character versions?

I think that's worth doing.

That class exists to allow JSON serialisation of versions across PowerShell versions, so it's not really necessary in the tests.

Thanks for fixing this @bergmeister!

[bergmeister]

OK, I will merge this and open a 2nd PR later to add the capability of parsing a single character version