Fix spark fixture

[ADBond]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Is your Pull Request linked to an existing Issue or Pull Request?

Previously test_helpers depended on the spark fixture. That means this gets created whenever we request the test_helpers fixture, which is slow (and particularly annoying if you're just running e.g. duckdb tests).

Give a brief description for the solution you have provided

Now both test_helpers and spark fixtures use a helper function, with the upshot that spark is only created in test_helpers whenever a SparkTestHelper is instantiated (which should only happen when you are running spark tests, due to LazyDict)

Currently this means that spark is created afresh each time we access a SparkTestHelper, which is not ideal.

PR Checklist

• Added documentation for changes
• Added feature to example notebooks or tutorial (if appropriate)
• Added tests (if appropriate)
• Updated CHANGELOG.md (if appropriate)
• Made changes based off the latest version of Splink
• Run the linter