Enable temporary .csproj creation and associated language server tests

[rmshaffer]

This PR re-enables temporary .csproj creation (essentially undoing #585) and also ensures that language server tests pass deterministically when opening both .qs files and non-.qs files.

There a few pieces to this fix, in addition to re-adding the code that was removed in #585:

1. Only create temporary projects for .qs files. Some BasicFunctionality tests load randomly-named source files without a .qs extension, and we want to allow those tests to operate as they do today.

2. Ensure that MSBuildLocator is registered for both the BasicFunctionality and ProjectLoader test classes via AssemblyInitialize attribute, rather than just a static constructor. With the static constructor approach, the order of execution was not deterministic, and if the BasicFunctionality tests were run prior to the ProjectLoader test class initialization, the proper version of MSBuild was not always being discovered. This actually led the tests to always pass, because the failure to create the temporary project was simply logged and the tests continued down their old path without a temporary project.

3. Change the temporary project folder naming scheme so that it is stable. The previous GetHashCode() approach was not even stable within a single test run, which occasionally caused errors. This is a good change from a user perspective as well, since it will ensure that the same temporary project is always used for code in a given folder.

[msoeken]

Looks good to me, but I am not the expert for this one.

[rmshaffer]

@ricardo-espinoza @bettinaheim the tests are now passing reliably with the current set of changes, so I think it is ready to go, pending your review. Thanks!