tests: Convert ExpressionMatchTests into a GTest-based test case

As first contender for the newly introduced test framework, use
the recently added ExpressionMatchTests class. Convert to use
GTest, and move into the new home for Quassel unit tests (the
tests/<module>/ directory).

With this, "make test" (or "ninja test") actually does something
if Quassel is configured with -DBUILD_TESTING=ON.