Fixed :meth:`~DecimalNumber.string_to_mob` for opengl and added tests

[k4pran]

Overview: What does this pull request change?

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

This is another batch of tests reused from cairo with the opengl renderer. These are grouped together as they were all failing due to string_to_mob_map holding objects from the cairo renderer that were being reused by opengl. I created a separate map for the opengl renderer to use so the tests can pass now.

Links to added or changed documentation pages

Further Information and Comments

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

[hydrobeam]

Was the old string_to_mob_map implementation causing errors just for the tests or was it for opengl generally?

[k4pran]

Was the old string_to_mob_map implementation causing errors just for the tests or was it for opengl generally?

Only for tests that I am aware of, think to create a runtime issue you might have to change renderer during a single run or something

[MrDiver]

Can we resolve the issues on this? Else i would suggest closing it