Improve code coverage with unit tests

[Lucas-C]

Current code coverage can be visualized here: https://app.codecov.io/gh/py-pdf/fpdf2

Goal: raise code coverage by one point

Our test strategy & tools (pytest) are described in the documentation

There are several minor cases (exceptions, unfrequent code paths...) that are not currently covered.
The codecov.io report allows to easily spot code lines not already covered by unit tests.
Example: rect_clip(), cf. issue #277

Any PR that raises code coverage is welcome 😊

Introducing hypothesis to cover the most-used FPDF methods would also be a very nice addition.

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By contributing those improvements, you, as a benevolent FLOSS developper, will help the fpdf2 users community by making this library more robust & reliable.
Moreover, by working on this task, you will learn about PDFs syntax and the lifecycle & structure of a popular Python library.
You will also be added into the contributors list & map.

[Lucas-C]

Given that code coverage has improved a lot since I created this issue 2 years ago, I'm going to close this now.