Use `GTESTS` instead of `PLAIN_TESTS_CXX`

Use `GTESTS` instead of `PLAIN_TESTS_CXX`

The former is from googletest.test.mk, whereas the latter is from plain.test.mk.

As noted in r344328, Kyua will adopt more native GoogleTest support. Thus, it's
more desirable to make the test interface more of an opaque blackbox for the
testcase implementer.