Add basic tests for reading/writing structure files

For each of the supported format (except for tpr), make a simple
roundtrip test, starting from a dummy topology, and doing a
write-read-write cycle, checking that the output files remain the same.
Additional tests for preservation of various properties of the topology
could also be added later.

Test both read_tps_conf() and read_stx_conf() separately for now.

Added to support refactoring of the code.

Change-Id: Idbdfb4fc39b124d9236a54896ef4613ebfe80ceb