Add element highlighting by default to the tests

[mshriver]

Makes it easier to observe tests when making contributions

Blocked by #158

Demonstrates the issue as well, with the check showing the following failure just from enabling highlighting:

=========================== short test summary info ============================
FAILED testing/test_chartpie.py::test_pie_chart[bottom-aligned] - AssertionError: assert 'rgb(0, 0, 0)' == 'rgb(0, 149, 150)'
  - rgb(0, 149, 150)
  ?        ^^^  --
  + rgb(0, 0, 0)
  ?        ^
FAILED testing/test_chartpie.py::test_pie_chart[right-aligned] - AssertionError: assert 'rgb(0, 0, 0)' == 'rgb(0, 47, 93)'
  - rgb(0, 47, 93)
  ?        ^^  ^^
  + rgb(0, 0, 0)
  ?        ^  ^
= 2 failed, 234 passed, 3 skipped, 1 xfailed, 6 warnings in 533.65s (0:08:53) ==