cmake: remove IS_TEST handling

Depending on a path inside the Zephyr tree to determine if we are a test
does not scale. Also some samples were marked as TEST while they are
not, just to get some options defined for tests.

Idenitfying a test will be addressed in another patch introducing
CONFIG_TEST.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>