perf test: Skip syscall enum test if no landlock syscall

perf test: Skip syscall enum test if no landlock syscall

The perf trace enum augmentation test specifically targets landlock_
add_rule syscall but IIUC it's an optional and can be opt-out by a
kernel config.

Currently trace_landlock() runs `perf test -w landlock` before the
actual testing to check the availability but it's not enough since the
workload always returns 0.  Instead it could check if perf trace output
has 'landlock' string.

Fixes: d66763fed30f0bd8c ("perf test trace_btf_enum: Add regression test for the BTF augmentation of enums in 'perf trace'")
Reviewed-by: Howard Chu <howardchu95@gmail.com>
Link: https://lore.kernel.org/r/20250128170629.1251574-1-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>