[tests][mono] Fix DoCopyOverCopiedFile test on mono/osx
[tests][mono] Fix DoCopyOverCopiedFile test on mono/osx

This test depends on File.GetLastWriteTime*, for a symlink, to return
the timestamp for the symlink itself, but on mono/osx it returns that
for the target file instead.

Issue: https://github.com/mono/mono/issues/12479

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