Fixed broken unit tests in 'test_sqlcompletion.py' and made non-class type unit tests into class type

[geleems]

• test_json_rpc_contracts.py
  • Renamed get_test_baseline() into get_baseline() since it was recognized as a unit test case by unit test auto discovery
• tables.py
  • raise StopIteration was used previously, which is ok in Python2 but not in Python3. StopIteration is converted to RuntimeError, and raise StopIteration throws exception in Python3. This broke several unit tests in test_sqlcompletion.py running in Python3. Proper way for backward compatibility is using return statement. (https://www.python.org/dev/peps/pep-0479/)
• test_prioritization.py and test_rowlimit.py
  • Converted into class type unit tests
• test_sqlcompletion.py
  • Fixed broken unit tests caused by StopIteration RuntimeError in Python3
  • Fixed broken unit test that was disabled: test_sub_select_multiple_col_name_completion
  • Converted into class type unit tests

All tests are passed in both Python2 and 3.

[pensivebrian]