Split large files into smaller chunks

[lucianjon]

execution.go and execution_test.go were large enough where they would sometimes be unwieldy to work with.

This PR keeps execution.go for the tests that run the entire execution pipeline, adds execution_introspection_test.go for the introspection tests that go against the internal resolver, adds execution_helpers_test.go for the tests that run against a particular function in the execution pipeline. Then execution_result.go and execution_result_test.go for the functions that operate on the results post execution.

Couple other small style tidy ups around the place too.

[codecov-commenter]

Codecov Report

Merging #159 (882206a) into main (127c5a4) will increase coverage by 0.04%. The diff coverage is 72.64%.

@@            Coverage Diff             @@
##             main     #159      +/-   ##
==========================================
+ Coverage   70.63%   70.68%   +0.04%     
==========================================
  Files          25       26       +1     
  Lines        2670     2671       +1     
==========================================
+ Hits         1886     1888       +2     
+ Misses        664      663       -1     
  Partials      120      120              

Impacted Files	Coverage Δ
execution_result.go	71.77% <71.77%> (ø)
execution.go	83.66% <85.71%> (+4.86%)	:arrow_up:
auth.go	89.17% <0.00%> (+0.63%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 127c5a4...882206a. Read the comment docs.

[pkqk]

@lucianjon if you can resolve the conflicts on execution.go we can get this in.