Directly embed lib.TestPreInitState in js/common.InitEnvironment

[na--]

Instead of manually including most of its constituent parts, we can just embed the whole *lib.TestPreInitState in the struct. This will make any future changes much easier.

I also tried to move FileSystems and CWD in lib.TestPreInitState, so we can get rid of them in both cmd.loadedTest and common.InitEnvironment (at that point, we could basically replace it with lib.TestPreInitState directly):

k6/cmd/test_load.go

Lines 31 to 39 in c09919b

	// loadedTest contains all of data, details and dependencies of a loaded
	// k6 test, but without any config consolidation.
	type loadedTest struct {
	sourceRootPath string // contains the raw string the user supplied
	pwd string
	source *loader.SourceData
	fs afero.Fs
	fileSystems map[string]afero.Fs
	preInitState *lib.TestPreInitState

Unfortunately, FileSystems is complicated, since when we are running .js files, its elements contain the "cache-on-read" overlay filesystems on top of the real FS (and on HTTPs), while for .tar archives we substitute them with the filesystems in the archive:

k6/js/bundle.go

Lines 134 to 137 in c09919b

	return newBundle(piState, &loader.SourceData{
	Data: arc.Data,
	URL: arc.FilenameURL,
	}, arc.Filesystems, arc.Options, false)

So, I decided to leave this for a future PR. With some further refactoring of the js test loading, it should be possible to handle in a nicer way than it currently is. Potentially before #2975 🤔

[codecov-commenter]

Codecov Report

Merging #2999 (c8ff1e0) into master (988f2f6) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head c8ff1e0 differs from pull request most recent head ed32098. Consider uploading reports for the commit ed32098 to get more accurate results

@@            Coverage Diff             @@
##           master    #2999      +/-   ##
==========================================
+ Coverage   76.99%   77.02%   +0.02%     
==========================================
  Files         228      228              
  Lines       17050    17050              
==========================================
+ Hits        13128    13132       +4     
+ Misses       3079     3076       -3     
+ Partials      843      842       -1     

Flag	Coverage Δ
ubuntu	77.02% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
js/common/initenv.go	66.66% <ø> (ø)
js/bundle.go	84.57% <100.00%> (-0.16%)	⬇️
js/modulestest/runtime.go	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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