microbit: add missing qtest_quit() call

Without this call, the QEMU process is being left running which on
FreeBSD 13.2 at least, makes meson think the test is still running,
and thus execution of "make check" continues forever.

This fixes the regression introduced in:

  commit a9c9bbe
  Author: Chris Laplante <chris@laplante.io>
  Date:   Tue Aug 22 17:31:02 2023 +0100

    qtest: microbit-test: add tests for nRF51 DETECT

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1882
Signed-off-by: "Daniel P. Berrangé" <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230912184130.3056054-2-berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230914155422.426639-5-alex.bennee@linaro.org>

tests/qtest/microbit-test.c

@@ -434,6 +434,8 @@ static void test_nrf51_gpio_detect(void)
     g_assert_true(qtest_get_irq(qts, 0));
     qtest_set_irq_in(qts, "/machine/nrf51", "unnamed-gpio-in", 3, 0);
     g_assert_true(qtest_get_irq(qts, 0));
+
+    qtest_quit(qts);
 }
 
 static void timer_task(QTestState *qts, hwaddr task)