[travis] execute tests in parallel

.travis.yml

@@ -68,7 +68,7 @@ before_script:
     - if [[ "x${TESTS}" != "xheadercheck" ]]; then ${SRC_DCTRL} ${BLD} --only=${MY_MODULE} bexec ${BUILD_CMD} test_binaries_${TESTS}; fi
 
 script:
-    - if [[ "x${TESTS}" != "xheadercheck" ]]; then travis_wait ${SRC_DCTRL} ${BLD} --only=${MY_MODULE} bexec ctest -R ${TESTS//_/.+}; fi
+  - if [[ "x${TESTS}" != "xheadercheck" ]]; then travis_wait ${SRC_DCTRL} ${BLD} --only=${MY_MODULE} bexec ctest -j 2 -R ${TESTS//_/.+}; fi
     - if [[ "x${TESTS}" == "xheadercheck" ]]; then ${SRC_DCTRL} ${BLD} --only=${MY_MODULE} bexec ${BUILD_CMD} headercheck; fi
     # retry this step because of the implicated race condition in cloning and pushing with multiple builder running in parallel
     - if [[ "x${TESTS}" != "xheadercheck" ]]; then travis_retry $HOME/src/scripts/bash/travis_upload_test_logs.bash $HOME/src/${MY_MODULE}/${DUNE_BUILD_DIR}/dune/gdt/test/; fi