Add --merge-mode-functions=separate to gcovr call in CI (#3716)

Author: paulromano

.github/workflows/ci.yml

@@ -186,6 +186,7 @@ jobs:
             --gcov-ignore-errors source_not_found \
             --gcov-ignore-errors output_error \
             --gcov-ignore-parse-errors suspicious_hits.warn \
+            --merge-mode-functions=separate \
             --print-summary \
             --lcov -o coverage-cpp.lcov || true