tests/harness/runHarness.py: deduplicate error reporting code.

Also seems to have a performance improvement.
This is due to not needing to building up error message strings for tests that pass.

assert a == b, foo()

Make use of that foo is being called lazily.

$ python -m cProfile  runHarness.py en-ueb-g2-dictionary_harness.txt
...

Ran 99299 tests (99.26% success), with 735 failures and 0 errors.
Before refactor:
16618186 function calls (16616268 primitive calls) in 22.915 seconds
After refactor:
16127571 function calls (16125653 primitive calls) in 21.783 seconds
tests/harness/runHarness.py: variable renaming for readibility.
tests/harness/runHarness.py: Provide compact form of test failures.

See: http://liblouis.org/todo/runharness_should_provide_option_for_Condensed_output/
runHarness: enable --compact_output in Makefile
Merge branch 'master' into mhameed_runharness_compact_output
add news entry.
Merge branch 'mhameed_runharness_compact_output'

* mhameed_runharness_compact_output:
  add news entry.
  runHarness: enable --compact_output in Makefile
  tests/harness/runHarness.py: Provide compact form of test failures.
  tests/harness/runHarness.py: variable renaming for readibility.
  tests/harness/runHarness.py: deduplicate error reporting code.

Fix for todo item: Runharness should provide option for Condensed output