UnauthorizedAccessException in DirectoryAssert

[Charlie Poole]

Running on Linux, the NUnit test method IsWithinPassesWithTempPath threw an UnauthorizedAccessException while trying to
access a restricted directory.

This is because DirectoryAssert.IsWithin checks against _all_ existing paths under the expected root and can happen in any instance where the path is not accessible.

DirectoryAssert is already deprecated and this indicates that it's obsolescence should be accelerated. A simple solution would be to use one of the PathConstraints in its implementation so that no access to actual directories is required.