testsuite/test-weakdep: remove custom handling

testsuite/test-weakdep: remove custom handling

Currently test-weakdep is the only test which explicitly sets the kernel
module lookup directory and the modprobe.d location. It does so by
explicitly hard-coding the full path for both, thus effectively
bypassing the path handling done in our `LD_PRELOAD` module `path.so`.

Just use `kmod_new(NULL, NULL);` which will ensure that everything is
handled relatively to `TC_ROOTFS` defined further down in the test.

Note: this technically reduces our test coverage, although kmod_new() is
not the goal of this test and should be handled separately.

Noticed while removing the TESTSUITE_ROOTFS instances from DEFINE_TEST()

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/258
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>