Improve send_signal BPF test stability

Substitute sleep with dummy CPU intensive computation.
Finish aforemention computation as soon as signal was
delivered to the test process. Make the BPF code to
only execute when PID global variable is set

Signed-off-by: Mykola Lysenko <mykolal@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220308200449.1757478-3-mykolal@fb.com