run-tests: fix whitelist/blacklist with directories on Windows
The file name is resolved with `os.path.relpath()` in the `Test` constructor,
which yields `\` on Windows. That doesn't match the `/` separator when using
MSYS tools to build the list, and it isn't obvious that this is the problem
because directory separators can mostly be used interchangeably. The
`--test-list` argument already seems to be properly handled.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present