Undo ConditionalFact workaround.

[NTaylorMullen]

Due to microsoft/vstest#291 [ConditionalFact]/[OSSkipCondition]` result in test not running. We've worked around this by adding a runtime check of the OSPlatform.

Marker comments have been added that look like:

// Temporary work around: https://github.com/aspnet/FileSystem/issues/254.

Can also revert the initial commit to add the workarounds: ac5e3f2