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 <andrea.righi@canonical.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>