Change performance tests to calculate the minimum of the execution times

Issue/feature addressed

Performance tests were unreliable and sometimes failed.

Proposed solution

The minimum execution time from a series of runs is probably more representative of the capability of the algorithm than the average.

Implementation

This MR changes the calculation in the ExecutionTime metric to use the minimum.

Tests

Suggested reviewers

@CFD-Xing

Notes

Please add any other information that could be useful for reviewers.

Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).