[12.x] reset Str factories when tearing down test

[cosmastech]

This restores the factories on the Str class to their defaults when tearing down test cases. I can't think of a reason this would really be a breaking change, but happy to be corrected.

[taylorotwell]

@cosmastech could someone be faking the str generation in a setUp method or something which this would break?

[cosmastech]

@cosmastech could someone be faking the str generation in a setUp method or something which this would break?

@taylorotwell, not exactly. The setUp runs before every test, but they could have it modified in setUpBeforeClass() or something. Or they could have written their tests with the expectation that modifying it in one case will always keep that going forward.

I can target master for this if you're ok with the change, but just want to avoid a BC on a minor version.

Could also probably call Str::flushCache() here.

[taylorotwell]

Yeah I think since it could be a breaking change can't do it on 12.x.