Skip overwrite test when root (#3471)
Skip overwrite test when root (#3471)

The `root` user has special permission to ignore the read-only bit, and
does so silently and without error. That caused this test to fail,
because we weren't erroring when attempting to overwrite a readonly
file.

Skip the test when running as root.
rainersigwald/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,chipitsine/msbuild,mono/msbuild,Microsoft/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,chipitsine/msbuild,cdmihai/msbuild,mono/msbuild,cdmihai/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,mono/msbuild,jeffkl/msbuild,cdmihai/msbuild,rainersigwald/msbuild,Microsoft/msbuild,Microsoft/msbuild,jeffkl/msbuild,chipitsine/msbuild,cdmihai/msbuild,Microsoft/msbuild,mono/msbuild