ci: Refactor CI scripts

1. CI_TARGET now determines which run_${CI_TARGET}.sh script to use. Defaults to
   `tests`.
2. Build no longer halts on the first failing suit: e.g. if functional tests
   failed it will continue with unit tests, etc.
3. All ${MAKE_CMD} occurrences moved to `top_make` function, added `build_make`
   as an alias to `make -C build` (`"${MAKE_CMD}" -C "${BUILD_DIR}"`) which is
   too verbose.

`suite.sh` was copied from powerline (tests/common.sh file), assumes running
with POSIX shells (and actually uses dash in powerline). Then some convenience
functions were added (run_test and below).

.travis.yml

@@ -50,6 +50,7 @@ env:
     - SUCCESS_MARKER="$BUILD_DIR/.tests_successful"
     # default target name for functional tests
     - FUNCTIONALTEST=functionaltest
+    - CI_TARGET=tests
 
 matrix:
   include: