Review time spent in nightly test suite

### User Story

As a developer of Sleeper, I want the nightly performance test suite to be as quick as possible, so that I can run it during the day when needed and get feedback on the system.

### Description / Background

We've recently noticed that the nightly performance tests are taking significantly longer than they did when we first parallelised it into multiple suites. It's possible this may be caused by this change to clear some build artefacts every night:
- https://github.com/gchq/sleeper/issues/7294

We'd like to review how long the nightly tests are taking, and see what we could do to improve it, e.g. by moving tests between the suites to reduce the time of the slowest test suite.

### Technical Notes / Implementation Details

The nightly tests run through the script `scripts/test/nightly/startNightlyTestsFromHost.sh`.

Start with scripts/test/nightly/startNightlyTestsFromHost.sh and run or inspect how it launches the nightly suites and records their durations. Review issue #7294 for the build-artifact cleanup change, then compare suite timings to identify the slowest suite. Done means documenting the cause and making or recommending changes that reduce the nightly test time.

Written by the indexing model from the issue text.