tests: mem_protect/demand_paging: wait a bit for NRU to work

This waits a bit for NRU eviction algorithm (which is the default)
to work its magic to clear the access bit of physical frames.
This increases the number of clean pages which can be evicted,
to make sure the number of clean pages evicted is not zero, which
would cause an assertion.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

tests/kernel/mem_protect/demand_paging/src/main.c

@@ -149,6 +149,10 @@ void test_touch_anon_pages(void)
 	zassert_not_equal(stats.eviction.dirty, 0UL,
 			  "there should be dirty pages being evicted.");
 
+#ifdef CONFIG_EVICTION_NRU
+	k_msleep(CONFIG_EVICTION_NRU_PERIOD * 2);
+#endif /* CONFIG_EVICTION_NRU */
+
 	/* There should be some clean pages to be evicted now,
 	 * since the arena is not modified.
 	 */