fix(snapshot): fixed snapshots always created in testza directory

[TobiaszCudnik]

Snapshots work only within testza itself and when used in another module, try to create a fixture file within the testza dir (which results in no permission).

This PR fixes a nesting issue in the snapshot support by extracting internal logic to a shared function (thus normalizing the stack trace).

Affected:

• SnapshotCreateOrValidate
• SnapshotCreate

PS. A regression test calling from another module would be nice, but not sure how to hook it up to the main go test.

[codecov]

Codecov Report

Merging #65 (8944be1) into main (e46e881) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   88.56%   88.64%   +0.07%     
==========================================
  Files          10       10              
  Lines         621      625       +4     
==========================================
+ Hits          550      554       +4     
  Misses         37       37              
  Partials       34       34              

Impacted Files	Coverage Δ
snapshot.go	84.84% <100.00%> (+0.97%)	⬆️
utils.go	77.77% <100.00%> (ø)

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[MarvinJWendt]

LGTM! Thanks for the fix! 🚀