test: Add CoreServices "smoke test" to check if initialization works

Before, CoreServices would cause a debug assertion when initializing it
without other stuff from `MixxxMainWindow`. This is just a smoke test to
verify that it's possible to initialize the `CoreServices` class on its
own, without `MixxxMainWindow.

CMakeLists.txt

@@ -1503,6 +1503,7 @@ add_executable(mixxx-test
   src/test/controller_mapping_validation_test.cpp
   src/test/controllerscriptenginelegacy_test.cpp
   src/test/controlobjecttest.cpp
+  src/test/coreservicestest.cpp
   src/test/coverartcache_test.cpp
   src/test/coverartutils_test.cpp
   src/test/cratestorage_test.cpp

src/test/coreservicestest.cpp

@@ -0,0 +1,32 @@
+#include <gtest/gtest.h>
+
+#include "coreservices.h"
+#include "test/mixxxtest.h"
+#include "util/cmdlineargs.h"
+
+class CoreServicesTest : public MixxxTest {
+};
+
+TEST_F(CoreServicesTest, TestInitialization) {
+    // Initialize the app
+    const int argc = 0;
+    CmdlineArgs cmdlineArgs;
+    cmdlineArgs.parse(argc, nullptr);
+
+    const auto pCoreServices = std::make_unique<mixxx::CoreServices>(cmdlineArgs);
+    pCoreServices->initializeSettings();
+    pCoreServices->initializeKeyboard();
+    pCoreServices->initialize(application());
+
+    EXPECT_NE(pCoreServices->getControllerManager(), nullptr);
+    EXPECT_NE(pCoreServices->getEffectsManager(), nullptr);
+    EXPECT_NE(pCoreServices->getGuiTick(), nullptr);
+    EXPECT_NE(pCoreServices->getLV2Backend(), nullptr);
+    EXPECT_NE(pCoreServices->getLibrary(), nullptr);
+    EXPECT_NE(pCoreServices->getPlayerManager(), nullptr);
+    EXPECT_NE(pCoreServices->getScreensaverManager(), nullptr);
+    EXPECT_NE(pCoreServices->getSettingsManager(), nullptr);
+    EXPECT_NE(pCoreServices->getSoundManager(), nullptr);
+    EXPECT_NE(pCoreServices->getVinylControlManager(), nullptr);
+    EXPECT_NE(pCoreServices->getVisualsManager(), nullptr);
+}