test: convert from ACT_EXT_TSYNC to syscall

Converts tests to use new seccomp syscall instead of prctl. Also removes
logic expecing side-effects from TSYNC failure.

Signed-off-by: Kees Cook <keescook@chromium.org>
test: clean up sibling exit defines

Make sure the sibling exit codes are reachable and easier to understand.

Signed-off-by: Kees Cook <keescook@chromium.org>
test: check that fixture can detect prctl errors

This tests that prctl errors in threads will be reported correctly.

Signed-off-by: Kees Cook <keescook@chromium.org>
test: add test for mode locking

Make sure the seccomp mode can't change.

Signed-off-by: Kees Cook <keescook@chromium.org>