Fix xunit warnings

[sean-gilliam]

This PR fixes a number of issues caught by the xunit analyzer.

• xunit1013 - these were false positives due to how we run the MNTK tests
• xunit1014 - basically changing hardcoded string references to nameof
• xunit2000 - basically changing the order so that expected value is first
• xunit2003 - use the xunit Null() check instead of Equals()
• xunit2004 - use the xunit True()/False() check instead of Equal()
• xunit2009 - use the xunit Contains()/StartsWith() check instead of Equals() + string methods
• xunit2012 - use the xunit Contains() check instead of Equals() + Any()

Anecdotal data, but I was able to shave off a minute from the build time and the build log was a bit nicer to look at.

[Aaronontheweb]

nice work @sean-gilliam