Fix random seed on tests

[jsbrittain]

Description

Some tests in the unit and integration suites call functions that make use of random variables, or include random variables in their calls (specifically numpy.random). This can lead to tests sometimes failing due to reasons that may be unrelated to recent code changes, as outlined (and demonstrated) in issue #2833.

Fixes #2833

Type of change

Fixing the numpy random seed within the test routines produces consistent behavior. However, since stochastic elements are present in the PyBaMM codebase in relation to both the Casadi and Idaklu solvers, extensive dependencies on these functions within the test suite are likely to be widespread and opaque. In order to fully rectify the issue (whilst maintaining the same simplicity in test set-up moving forward) a metaclass solution is proposed.

The proposed solution wraps all methods that inherit from TestCase with a wrapper function that fixes the random seed to a hash code derived from the method's name. This permits hashes to be changed in problematic circumstances by a hash modifier, such as appending a trailing underscore (or other modifier) to the method name.

Specifically, the TestCase class, which is described as the 'Custom TestCase class for pybamm' is given a custom metaclass. However, it was also noted that this test class is not in widespread use across the testbase, and hence unittest.TestCase has now been replaced with TestCase as imported from tests across all test scripts.

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all
• The documentation builds: $ python run-tests.py --doctest

You can run unit and doctests together at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (cc1bff5) 99.69% compared to head (5fa9e97) 99.69%.

❗ Current head 5fa9e97 differs from pull request most recent head 6f1a384. Consider uploading reports for the commit 6f1a384 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2844   +/-   ##
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  Coverage    99.69%   99.69%           
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  Files          273      273           
  Lines        19065    19065           
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  Hits         19006    19006           
  Misses          59       59           

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[valentinsulzer]

Looks good to me. Will need changelog update (moving to new section) and merging the changes from the casadi 3.6 update

[valentinsulzer]

Changelog updates are in the wrong month here