selftests/bpf: Guarantee that useep() calls nanosleep() syscall

selftests/bpf: Guarantee that useep() calls nanosleep() syscall

Some implementations of C runtime library won't call nanosleep() syscall from
usleep(). But a bunch of kprobe/tracepoint selftests rely on nanosleep being
called to trigger them. To make this more reliable, "override" usleep
implementation and call nanosleep explicitly.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Julia Kartseva <hex@fb.com>
Link: https://lore.kernel.org/bpf/20200311185345.3874602-1-andriin@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>