Instead, make the args property a parameter list, and add params to that.
This means Command's serialize method have a different signature compared
to Parameter's, letting us do some more interesting things with it.
* 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.
