85 issues with next version of ggplot2v3.4.0 rc

[pwep]

This is attempting to address #85 (and similar issues in the future.) Clearly the structure of ggplot objects has changed slightly compared to our saved test output, used for comparison. This proposed change does not test the content of the ggplot object, only that it is a ggplot object.

If this change is acceptable, there is then an opportunity to reduce the file size of the plotting test comparison files. Currently the tests/testthat/goodValues361/Plots directory is 2.1Mb

[codecov[bot]]

Codecov Report

Base: 72.99% // Head: 72.99% // No change to project coverage :thumbsup:

Coverage data is based on head (3d71dc5) compared to base (ad7add7). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## developer #86 +/- ## ========================================== Coverage 72.99% 72.99% ========================================== Files 82 82 Lines 9698 9698 ========================================== Hits 7079 7079 Misses 2619 2619 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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