testsuite: ztest: Increase ZTEST_TEST_DELAY_MS to 5000

This commit increases the default value of `ZTEST_TEST_DELAY_MS` from 3000
to 5000 milliseconds because the current value of 3000ms may not be
sufficient for the hosts with lower CPU clock frequency (e.g. new Zephyr CI
runners with cost-effective processors).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>