tests: couple of fixes

The testbench wasn't actully run during distcheck, this changeset turns it on.

There are files in tests/ that aren't exported in the tarball. I've added the ones that are actually used during make check, but maybe some of the remaining should be added.
Currently all files are enumerated twice - in EXTRA_DIST and TESTS. Maybe it would be more resilient to have partial lists (TESTS_GENERIC, TESTS_IMPTCP, etc) that would be included into TESTS and EXTRA_DIST. But that would need the contents of tests/Makefile.am to be heavily shuffled.

Currently, running make distcheck fails for me, but I have some idea of what needs to be changed.