Refactor output to file into separate function
add small data file for use in read_data tests

was created using the (new) `make_file` function in convert_data.py
add some actual tests

* openclose.c renamed open_close.c for consitency
* added header file read_data_tests.h
* open_close.c and open_nonexistent_file.c call library functions and perform
  tests
* clean up Makefile.am a bit