test/cmake: fix building a list of unit tests

Commit 2be74a65 ("test/cmake: add a function for generating unit
test targets") added a function for generating unit test targets in
CMake. However, due to a wrong scope UNIT_TEST_TARGETS remains empty
after generating unit tests. Patch fixes that.

NO_CHANGELOG=build infrastructure
NO_DOC=build infrastructure
NO_TEST=build infrastructure