Code fix unit tests: FileMayOnlyContainASingleClass

[sharwell]

Implement unit tests for #351.

[NikolayIT]

@sharwell What do we need to do here?
Add more tests to https://github.com/DotNetAnalyzers/StyleCopAnalyzers/blob/master/StyleCop.Analyzers/StyleCop.Analyzers.Test/MaintainabilityRules/SA1402UnitTests.cs?

[sharwell]

Right now we only test that the second class was removed from the original file. We need to also test that the contents were correctly added to a new file with the expected name. I'm not sure what the best way to do this is, but I would at least start by looking at the unit tests for SA1649.

[bjornhellander]

PR #2140 added check of the newly created file's content. I can have a go at also checking the file name.