Merge branch 'feature/ci-run-parallel' into 'master'

[ci] run tests in parallel

See merge request tools/frackit!176

.ci_run_tests

@@ -19,7 +19,7 @@ mkdir build && cd build && cmake -DCMAKE_C_COMPILER="${CI_COMPILER}-${CI_COMPILE
                                  -DCMAKE_CXX_COMPILER="${CI_CXX_COMPILER}-${CI_COMPILER_VERSION}" ../
 
 # build tests
-make build_tests
+make -j4 build_tests
 
 # run them
-ctest -V
+ctest -j4 -V