chore(linux): Check existence of services directory

When running the integration tests, we provide a test service that
replaces `keyman-system-service`. DBus needs the updated `.service`
files in the `services` directory so that it will use the test
service. This change outputs an error and returns an non-zero
exit code if it can't find the `services` directory, which usually
means that `keyman-system-service` didn't get build.

linux/ibus-keyman/tests/KmDbusTestServer.cpp

@@ -139,6 +139,11 @@ void KmDbusTestServer::Loop()
 
 int
 main(int argc, char *argv[]) {
+  if (!g_file_test(KEYMAN_TEST_SERVICE_PATH, G_FILE_TEST_IS_DIR)) {
+    std::cerr << "ERROR: Directory " << KEYMAN_TEST_SERVICE_PATH << " doesn't exist! Exiting." << std::endl;
+    return 1;
+  }
+
   KmDbusTestServer testServer;
   testServer.Loop();