[REM] core: assertion reports

That's a not-very-useful subset of OdooTestResult, so:

* make results merge-able (aka add ability to update a result with the
  contents of another)
* remove support for test data files, and transmission of the
  assertion report thing through the data-files loading
* replace "legitimate" uses of assertion report by test result
* have run_unit_tests manipulate and return a result instead of weird
  flags & ternaries