[tests] Fix XMakeAppTests.GetCommandLineQuotedExe
[tests] Fix XMakeAppTests.GetCommandLineQuotedExe

- Quoting the msbuild executable filename, on Unix fails as it is being
  used directly with Process class, with no shell involved. So, it fails
  as the `\"/path/to/msbuild.dll\"` doesn't exist.

mono/msbuild,mono/msbuild,mono/msbuild,mono/msbuild