Add coverage tools (lcov, fastcov) to proxysql-ci-base Docker image

- Add lcov package for coverage report generation
- Install fastcov via pip3 for faster coverage processing
- Required for COVERAGE=1 mode in multi-group test runner

test/infra/docker-base/Dockerfile

@@ -26,6 +26,8 @@ RUN apt-get update -qq && \
     python3-structlog \
     php-cli \
     php-mysql \
+    lcov \
+    && pip3 install --break-system-packages fastcov \
     && wget https://github.com/openark/orchestrator/releases/download/v3.2.6/orchestrator-client_3.2.6_amd64.deb -O /tmp/orc.deb \
     && apt-get install -y /tmp/orc.deb \
     && rm -rf /var/lib/apt/lists/* /tmp/orc.deb