* test/api/*.vala: Renamed files that contained mock objects to *-mock.vala,
not *-test.vala.
* test/testcase.vala: Renamed to test-case.vala for consistency, remove
TestCase class from Gee package since that's really not true. Clean up
code for consistency.
* test/meson.build, test/CMakeLists.txt: Split TestCase compilation out
into a separate test lib.