kselftest/runner: allow to properly deliver signals to tests

While running seccomp_bpf, kill_after_ptrace() gets stuck if we run it
via /usr/bin/timeout (that is the default), until the timeout expires.

This is because /usr/bin/timeout is preventing to properly deliver
signals to ptrace'd children (SIGSYS in this case).

This problem can be easily reproduced by running:

$ sudo make TARGETS=seccomp kselftest
...

# [ RUN ] TRACE_syscall.skip_a#
not ok 1 selftests: seccomp: seccomp_bpf # TIMEOUT

The test is hanging at this point until the timeout expires and then it
reports the timeout error.

Prevent this problem by passing --foreground to /usr/bin/timeout,
allowing to properly deliver signals to children processes.

Signed-off-by: Andrea Righi <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/kselftest/runner.sh

···
 {
 	# Make sure tests will time out if utility is available.
 	if [ -x /usr/bin/timeout ] ; then
-		/usr/bin/timeout "$kselftest_timeout" "$1"
+		/usr/bin/timeout --foreground "$kselftest_timeout" "$1"
 	else
 		"$1"
 	fi