runners: tests: fix os.path.isfile patch

Copy a fix from test_nrfjprog.py to the other runner test suites. The
current code will enter an infinite recursion if you hit the path
where os.path.isfile is called, since it's been patched to
os_path_isfile_patch in the calling context. The fix is to cache the
'real' version in the parent scope and call it directly as a fallback.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
4 files changed