twister: remove fallback testcase if cases added at runtime
twister: remove fallback testcase if cases added at runtime

The fallback testcase, which is the scenario name from the yaml file is
added if we did not discover any testcases via parsing. It can be
removed if we discover testcaes at runtime or otherwise it is added as
skipped...

Signed-off-by: Anas Nashif <0d9952ec84ac43c159f6b7e7ed99a9080c00dd6e@intel.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr