Generate baseline hashes during tests to reduce flakiness

[ConorMacBride]

• The pytest-mpl test suite relies on lots of hardcoded hashes remaining the same, which leads to lots of test failures on different platforms.
• Instead of hardcoding the hashes we can quickly generate them during each test run and then modify specific hashes to simulate an expected test failure.

Closes #232 Closes #225 Closes #179