Create a line length test helper

A line length test helper will take a chunk of source , and a regex, and verify the bear accepts source under the line length and rejects lines over the line length.

The max_line_length setting can be automatically determined by the lines in the source provided by the caller. The bear should pass this test.

The regex is then used to increase the size of the source line lengths by 1 character, in accordance with the source language rules.

The helper confirms that the source was increased by 1 character only, and re-runs the bear, and expects it to fail.