tests: ztest: add CONFIG_ZTEST_NO_YIELD to keep SOC awake after test

Some SOCs cannot be flashed reliably in low power modes. If
CONFIG_ZTEST_NO_YIELD is selected, do not yield to the idle thread after
testsuite completes, so that the SOC will not enter low power mode.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>

subsys/testsuite/ztest/Kconfig

@@ -79,6 +79,14 @@ config ZTEST_ASSERT_HOOK
 	  error test case. Remember to add ignore_fault tag in yaml file when
 	  using twister to run testing.
 
+config ZTEST_NO_YIELD
+	bool "Do not yield to the idle thread after tests complete"
+	help
+	  When the tests complete, do not yield to the idle thread and instead
+	  spin in a loop. This is useful for low power mode tests, where
+	  yielding to the idle thread may put the board into a low power state
+	  where a debugger cannot connect to it.
+
 if ZTEST_NEW_API
 
 menu "ztest provided rules"

subsys/testsuite/ztest/src/ztest.c

@@ -579,5 +579,17 @@ void main(void)
 			state.boots = 0;
 		}
 	}
+#ifdef CONFIG_ZTEST_NO_YIELD
+	/*
+	 * Rather than yielding to idle thread, keep the part awake so debugger can
+	 * still access it, since some SOCs cannot be debugged in low power states.
+	 */
+	uint32_t key = irq_lock();
+
+	while (1) {
+		; /* Spin */
+	}
+	irq_unlock(key);
+#endif
 }
 #endif