test: don't skip acceptance tests when targeted with -run

[G-Rath]

This is a neat little trick I thought of while cleaning up the snapshots - all its saving us from is having to stick TEST_ACCEPTANCE=true in front of our go test calls, but I think it's nice to have the test suite not require that when you're running a specific acceptance test.

Credit to go-snaps for proving it must be possible, since this is what it does to determine if a snapshot is obsolete or that the tests have just not been run.

I've gone with the slightly awkward name isThisTestRunTarget as I felt it was less ambiguous than isThisTestRunning and similar, since yes the test must be running if the function is being called... 😅

[cuixq]

the flag is run or test.run?

[G-Rath]

test.run, as that's actually what is used under the hood I believe because really things are being compiled into a binary, so this prefix helps avoid clashes with tests & binaries that include flags.

From the docs:

Each of these flags is also recognized with an optional 'test.' prefix,
as in -test.v. When invoking the generated test binary (the result of
'go test -c') directly, however, the prefix is mandatory.

[another-rex]

Neat! Hopefully this means I can debug an acceptance test in vscode without having to comment out the "skip if not acceptance test" function now.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 68.75%. Comparing base (3bd1565) to head (860bbc8).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/testutility/utility.go	12.50%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1630      +/-   ##
==========================================
- Coverage   68.77%   68.75%   -0.03%     
==========================================
  Files         199      199              
  Lines       18949    18956       +7     
==========================================
+ Hits        13032    13033       +1     
- Misses       5221     5227       +6     
  Partials      696      696              

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