selftests: drv-net: add helper for path resolution

selftests: drv-net: add helper for path resolution

Refering to C binaries from Python code is going to be a common
need. Add a helper to convert from path in relation to the test.
Meaning, if the test is in the same directory as the binary, the
call would be simply: cfg.rpath("binary").

The helper name "rpath" is not great. I can't think of a better
name that would be accurate yet concise.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20250207184140.1730466-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>