tests/qtest: skip m48t59-test if the machine is absent

Together with the series at https://patchew.org/QEMU/20240423131612.28362-1-pbonzini@redhat.com/,
this allows adding sparc-softmmu to the target list of the
build-without-defaults CI job.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/qtest/m48t59-test.c

@@ -262,11 +262,12 @@ int main(int argc, char **argv)
     base_setup();
 
     g_test_init(&argc, &argv, NULL);
-
+    if (qtest_has_machine(base_machine)) {
         if (g_test_slow()) {
             /* Do not run this in timing-sensitive environments */
             qtest_add_func("/rtc/bcd-check-time", bcd_check_time);
         }
         qtest_add_func("/rtc/fuzz-registers", fuzz_registers);
+    }
     return g_test_run();
 }