It only runs run_tests.sh and checks coverage, which can be replaced by
just moving the coverage check to ci/run_tests.sh.

It's a leftover artifact that currently just acts as an unnecessary
intermediary script that calls the Makefile. It can be replaced by just
calling the Makefile directly.

- Move __gcov_flush to process_spawn, for more reliable coverage
  tracking of subprocesses
- Travis: use GCOV_ERROR_FILE
- codecov: use "-X fix" to skip "fixing" uploaded coverage data; it
  should be handled by codecov's backend instead.
- AppVeyor: no $PATH mangling, which breaks with the improved coverage tracking
  due to missing .dll in PATH.

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).