The VFS tests were using Unix absolute paths, which does not play well
when Windows see them as relative to the current drive letter.
By using the temporal directory, both Windows and Unix can use the same
paths and avoid the problem.
Additionally, a couple of inputs have to be transformed into the native
path format, because sourcekitd-test compares the inputs as strings, and
they need to match exactly. So the source file and the name of the VFS
entries are transformed into native using the helper from LLVM support.
Using key.sourcefile to provide the contents of key.sourcetext is meant
for testing, and does not have a straightforward meaning in the future
when the documents are combined with the VFS. Make it an error to mix
this with a VFS for now.
