selftests/bpf: Fix two memory leaks in prog_tests

Fix log_fp memory leak in dispatch_thread_read_log.
Remove obsolete log_fp clean-up code in dispatch_thread.

Also, release memory of subtest_selector. This can be
reproduced with -n 2/1 parameters.

Signed-off-by: Mykola Lysenko <mykolal@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220428225744.1961643-1-mykolal@fb.com