Add tests for the UsernameSet and Prefix classes

How to build test:
mkdir build_test
cd build_test
cmake -DBUILD_TESTS=ON ..
make -j
Then to run the tests, use either:
make test
ctest
./chatterino-test

tests/src/main.cpp

@@ -0,0 +1,3 @@
+#include <gtest/gtest.h>
+
+// do nothing