MSBuild task tests fail when long-paths enabled

[rainersigwald]

This test

https://github.com/Microsoft/msbuild/blob/94e11e0a773bc8956caf128335433231bb06fed5/src/Build.UnitTests/BackEnd/MSBuild_Tests.cs#L30-L93

Fails when long paths are enabled.

I think the reason is that there is a codepath that attempts to handle too-long-but-can-be-simplified-to-short-enough paths that has different behavior than actually just passing the long paths.

https://github.com/Microsoft/msbuild/blob/94e11e0a773bc8956caf128335433231bb06fed5/src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/MSBuild.cs#L314

Disabling the tests against this bug.