TSCTestSupport: avoid dropping path components

[compnerd]

Given that the AbsolutePath constructor invoked here is
AbsolutePath(_:relativeTo:) dropping the first component is not
meaningful for UNIX paths as an absolute path relative to root is going
be the same path as the path relative to the root being re-formed to be
relative to the root. However, this meaningfully impacts paths on
Windows as the shared path representation currently does not include the
drive or server & share (in the UNC case) and the resulting absolute
path representation is a drive-relative path, where dropping the first
component alters the path representation.

[compnerd]

@swift-ci please test

[tomerd]

done have tests to make sure this still works correctly on all platforms?

[compnerd]

The tools-support-core tests don't work on Windows (I'm tempted to spend the necessary energy towards removing the dependency). The SPM tests will definitely exercise this heavily, and the assertion in AbsolutePath (i.e. path.pathString.utf8.first! == "/"). I think that this should be sufficiently covered. But, if there is something that you think might be useful to add - I can try to add something blind.