tests: settings: remove passing -DTEST_${TEST} from cmake

This feature is not used in most tests, as there is no logic in C code that
utilizes `#ifdef TEST_x` statement.

The only test that does that is `tests/subsys/settings/fcb/` where `#ifdef
TEST_LONG` is used, so leave that test case untouched.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>