Create Test Package

[theunrepentantgeek]

What this PR does / why we need it:

Eliminates most duplication of test support infrastructure, allowing reuse across multiple packages.

Doesn't eliminate all copies of FakeFunction as we can't move the uses within astmodel out without causing a package reference cycle.

Special notes for your reviewer:

Will be making more reuse of methods in an upcoming PR, testing functions.

How does this PR make you feel:

[babbageclunk]

Looks good!

[babbageclunk]

super minor: I have a slight preference for ReturnType here.
Oh, I see that you did this to make it consistent with Imported and Referenced. But actually present tense (Imports, References) feels more natural for those too, and I quite like that they can also be read as plural nouns as well since they're sets. 🤷

[theunrepentantgeek]

Also, the function ReturnType() is on the Function interface, so I couldn't use ReturnType for the member name here.

[codecov-commenter]

Codecov Report

Merging #1567 (0b6c838) into master (a4d3a51) will increase coverage by 0.00%.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master    #1567   +/-   ##
=======================================
  Coverage   63.41%   63.41%           
=======================================
  Files         178      181    +3     
  Lines       11739    11793   +54     
=======================================
+ Hits         7444     7479   +35     
- Misses       3628     3645   +17     
- Partials      667      669    +2     

Impacted Files	Coverage Δ
hack/generator/pkg/test/asserts.go	71.42% <71.42%> (ø)
hack/generator/pkg/test/file_definition.go	77.77% <77.77%> (ø)
.../codegen/pipeline_add_cross_resource_references.go	91.76% <100.00%> (+0.62%)	⬆️
hack/generator/pkg/test/fake_function.go	53.12% <100.00%> (ø)

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