selftests/bpf: Fix issues in parse_num_list()

The function does not check that parsing_end is false after parsing
argument. Thus, if the final part of the argument is something like '4-',
which is invalid, parse_num_list() will discard it instead of returning
-EINVAL.

Before:

 $ ./test_progs -n 2,4-
 #2 atomic_bounds:OK
 Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

After:

 $ ./test_progs -n 2,4-
 Failed to parse test numbers.
Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220406003622.73539-1-ytcoode@gmail.com

tools/testing/selftests/bpf/testing_helpers.c

@@ -60,7 +60,7 @@ int parse_num_list(const char *s, bool **num_set, int *num_set_len)
 			set[i] = true;
 	}
 
-	if (!set)
+	if (!set || parsing_end)
 		return -EINVAL;
 
 	*num_set = set;