selftests: bpf: test_progs: initialize duration in singal_pending test

CHECK macro implicitly uses duration. We call CHECK() a couple of times
before duration is initialized from bpf_prog_test_run().
Explicitly set duration to 0 to avoid compiler warnings.

Fixes: 740f8a65 ("selftests/bpf: make sure signal interrupts BPF_PROG_TEST_RUN")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>