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

CMakeLists.txt

@@ -0,0 +1,37 @@
+cmake_minimum_required(VERSION 3.8)
+
+project(chatterino)
+
+include_directories(src)
+
+set(chatterino_SOURCES
+    src/common/UsernameSet.cpp
+    )
+
+find_package(Qt5Widgets CONFIG REQUIRED)
+find_package(Qt5 5.9.0 REQUIRED COMPONENTS
+    Core
+    )
+
+# set(CMAKE_AUTOMOC ON)
+
+if (BUILD_TESTS)
+    message("++ Tests enabled")
+    find_package(GTest)
+    enable_testing()
+
+    add_executable(chatterino-test
+        ${chatterino_SOURCES}
+
+        tests/src/main.cpp
+        tests/src/UsernameSet.cpp
+        )
+
+    target_link_libraries(chatterino-test Qt5::Core)
+
+    target_link_libraries(chatterino-test gtest gtest_main)
+
+    gtest_discover_tests(chatterino-test)
+else()
+    message(FATAL_ERROR "This cmake file is only intended for tests right now. Use qmake to build chatterino2")
+endif()