cmake: set IS_TEST in the samples that were including Makefile.test

For an unknown reason, various samples in KBuild were including
Makefile.test, this had some desired benefits, one of which is that
the popular BOOT_BANNER appears. The CMake-equivalent of including
Makefile.test is setting the flag IS_TEST. This commit reverts the
behaviour of the samples back to how it was pre-cmake.

Signed-off-by: Sebastian Boe <sebastian.boe@nordicsemi.no>