Reuse CLI flag tests for OpenGL

[k4pran]

Overview: What does this pull request change?

Motivation and Explanation: Why and how do your changes improve the library?

To help give us confidence during transition to opengl we can duplicate cairo CLI tests to improve our opengl test coverage and ensure current behaviour is consistent during transition

Links to added or changed documentation pages

Further Information and Comments

Below are the currently failing tests

Test Name	Comments
test_mp4_format_output	dependent on #1982
test_gif_format_output	dependent on #1982
test_a_flag	dependent on #2011
test_images_are_created_when_png_format_set	dependent on #1982
test_images_are_zero_padded_when_zero_pad_set	dependent on #1982
test_webm_format_output	dependent on #1982
test_default_format_output_for_transparent_flag	dependent on #1982
test_mov_can_be_set_as_output_format	dependent on #1982

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[Darylgolden]

LGTM