[REF] Simplify getMembershipStatusByDate more

[demeritcowboy]

Overview

Follow-on from #18030

Before

Hard to understand function

After

A little better

Technical Details

• Added/updated comments
• Convoluted way of doing date('Ymd')
• Required param in the middle of param list doesn't need to be required, and accepted null before anyway.
• Move variables only used inside inner if into inner if to save valuable nanoseconds.
• Add more test
• There's some other tests in the file that don't clean up after themselves but need a stock setup to compare. Cleaned those up.

Comments

Could spend all day filling out the data provider. Also there's probably more in this function to simplify.

To see the test runs the same before and after you'd need to make the one change in the function declaration so the parameter isn't required, but otherwise it's the same result before and after.

[civibot]

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[eileenmcnaughton]

@demeritcowboy thanks for doing this - it hurt to read but it hurts much less afterwards