[Test framework] re-re-fix test and add test for test

[demeritcowboy]

Overview

Thanks @eileenmcnaughton for the test update in #18009 - I didn't think of that and it makes sense. But in September it comes out as "2020-13-27" (on windows anyway).

Before

Test fix in #18009 addressed an issue in the previous test fix but added new issues.

After

Hopefully no more issues.

Technical Details

And in January the month string would be '5' without the leading 0.

Also the '+6 months' lower down needs a similar treatment.

Comments

Nothin' but test.

[civibot]

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[seamuslee001]

Looks good to me

[eileenmcnaughton]

@demeritcowboy nice test that whole membership calc dates needs a dataprovider test like this & a lot of simplification!