test: skip ptrace if SINGLESTEP is not supported

On armv7hl the PTRACE_SINGLESTEP call is no longer supported. Make sure
we skip the test in this case.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

src/test-parallel.c

@@ -284,6 +284,15 @@ static int test_parallel(void) {
 
                         /* step child */
                         r = ptrace(PTRACE_SINGLESTEP, pid, 0, 0);
+
+                        /*
+                         * Some architectures (e.g., armv7hl) do not implement
+                         * SINGLESTEP, but return EIO. Skip the entire test in
+                         * this case.
+                         */
+                        if (r < 0 && errno == EIO)
+                                return 77;
+
                         assert(r >= 0);
                         break;