test: make createTestDir a general test utility

[G-Rath]

We've already got this duplicated in two places, and I have need for it in an upcoming PR and @robramsaynz should be using it in #797 so let's move this into testutility

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 59.70%. Comparing base (b28c1c8) to head (8c427dc).
Report is 2 commits behind head on main.

Files	Patch %	Lines
internal/testutility/utility.go	72.72%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #832      +/-   ##
==========================================
- Coverage   59.78%   59.70%   -0.09%     
==========================================
  Files         136      136              
  Lines       11268    11279      +11     
==========================================
- Hits         6737     6734       -3     
- Misses       4102     4115      +13     
- Partials      429      430       +1     

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[spencerschrock]

Happened to see this while checking on another PR here.
t.Cleanup would be helpful here. no need to make caller remember to call cleanup function.

Or there's also just t.TempDir in the stdlib already

[G-Rath]

oh nice - and yeah I just realised t.TempDir is a thing though sadly it doesn't support a prefix which I personally like to easily identify and manually clean up in the case of a failure or debugging.

Oh well, still probably best to just switch to using that

[G-Rath]

ok after playing with t.TempDir I'm not going to use it for now because we need to be able to normalize the test directory for snapshots, which is too hard with t.TempDir right now 😞

[spencerschrock]

Sounds reasonable. At least t.Cleanup sounds useful in this case.

[another-rex]

Thanks!