test: avoid -Wunused-variable on NoAsserts build
Ensure that the variable is referenced to avoid an unused variable
warning from GCC/clang when building the tests in a Release+NoAsserts
configuration.  This is done as part of the swift build on Linux for
SourceKit.
@@ -128,6 +128,7 @@ main(int argc, char *argv[])
tv_wall.tv_usec = labs(tv_stop.tv_usec - tv_start.tv_usec);
int res2 = wait4(pid, &status, 0, &usage);
+ (void)res2;
assert(res2 != -1);
test_long("Process exited", (WIFEXITED(status) && WEXITSTATUS(status) && WEXITSTATUS(status) != 0xff) || WIFSIGNALED(status), 0);
printf("[PERF]\twall time: %ld.%06ld\n", tv_wall.tv_sec, tv_wall.tv_usec);
