Fix code coverage collection: mount gcov directory in test runner

The test runner container needs access to the /gcov directory where
ProxySQL writes its coverage data (.gcda files). Without this mount,
the coverage collection script could only see .gcno files (compile-time)
but not the runtime coverage data from ProxySQL.

This fix adds the gcov volume mount to the test runner container,
allowing it to merge coverage data from each ProxySQL instance.

test/infra/control/run-tests-isolated.bash

@@ -77,6 +77,7 @@ NETWORK_NAME="${INFRA_ID}_backend"
 TEST_CONTAINER="test-runner.${INFRA_ID}"
 INFRA_LOGS_PATH="${WORKSPACE}/ci_infra_logs"
 PROXY_DATA_DIR_HOST="${INFRA_LOGS_PATH}/${INFRA_ID}/proxysql"
+COVERAGE_DATA_DIR_HOST="${INFRA_LOGS_PATH}/${INFRA_ID}/gcov"
 
 
 
@@ -134,7 +135,8 @@ docker run \
     --cap-add=NET_ADMIN \
     --cap-add=SYS_ADMIN \
     -v "${WORKSPACE}:${WORKSPACE}" \
-        -v "${PROXY_DATA_DIR_HOST}:/var/lib/proxysql" \
+    -v "${PROXY_DATA_DIR_HOST}:/var/lib/proxysql" \
+    -v "${COVERAGE_DATA_DIR_HOST}:/gcov" \
     -e WORKSPACE="${WORKSPACE}" \
     -e INFRA_ID="${INFRA_ID}" \
     -e INFRA_TYPE="${INFRA_TYPE}" \