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).

ci/script.sh

@@ -3,16 +3,11 @@
 set -e
 set -o pipefail
 
-if [[ -n "${CI_TARGET}" ]]; then
-  make "${CI_TARGET}"
-  exit 0
-fi
-
 # This will pass the environment variables down to a bash process which runs
 # as $USER, while retaining the environment variables defined and belonging
 # to secondary groups given above in usermod.
 if [[ "${TRAVIS_OS_NAME}" == osx ]]; then
-  sudo -E su "${USER}" -c "ci/run_tests.sh"
+  sudo -E su "${USER}" -c "ci/run_${CI_TARGET}.sh"
 else
-  ci/run_tests.sh
+  ci/run_${CI_TARGET}.sh
 fi