selftests/bpf: drop serial restriction

Patch 1/2 added PID filtering to the probe_user BPF program to avoid
cross-test interference from the global connect() hooks.

With the interference removed, drop the serial_ prefix and remove the
stale TODO comment so the test can run in parallel.

Tested:
  ./test_progs -t probe_user -v
  ./test_progs -j$(nproc) -t probe_user

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

Author: ahjf-07

tools/testing/selftests/bpf/prog_tests/probe_user.c

@@ -1,8 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <test_progs.h>
 
-/* TODO: corrupts other tests uses connect() */
-void serial_test_probe_user(void)
+void test_probe_user(void)
 {
 	static const char *const prog_names[] = {
 		"handle_sys_connect",