make: tests: Add globally unified test target `make full-test`

This target in `tests/` checks if a target `test` exists in the Makefiles of
the subdirectories of the first level in `tests/` and executes them with
`make clean all flash test`
tests: Fix tests that were defect

* I used pexpect instead of when expect script was not working expect, since
  TCL confuses me
* I deleted the thread_exit test since it was completely invalid
travis: let travis execute all tests for native