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>