Refactor gui-side webrtc networking code, adding tests.

[jsoules]

This PR presents a few changes to the gui-side webrtc networking code, mainly to facilitate testing.

The following were the significant changes:

• Move utility functions--sleeping, random alpha strings--out of WebrtcConnectionToService.ts into their own files.
• Implement dependency injection design pattern for the WebrtcConnectionToService so that it no longer instantiates its "pet" SimplePeer object in the constructor. This is mainly to allow testing, but could facilitate replacement in the future. It also necessitates breaking out some of the chained actions in configuring the SimplePeer object and attempting to connect, which had been part of the constructor.
• Replacing some of the internal-constant timeout values with named constants.
• Update config.ts to use the new instantiation procedure.

Now has 100% test coverage documenting the expected behaviors of the gui-side networking. I've also confirmed via dev run that it still works--console with local service documents communication over the webrtc channel. There may be proper integration tests to add that will check that as well; I've been focused on detailed unit testing.

[codecov-commenter]

Codecov Report

Merging #78 (9b199a4) into main (50d8e8b) will increase coverage by 2.47%. The diff coverage is 95.55%.

@@           Coverage Diff            @@
##            main     #78      +/-   ##
========================================
+ Coverage   7.07%   9.54%   +2.47%     
========================================
  Files         72      74       +2     
  Lines       5652    5677      +25     
  Branches      78     112      +34     
========================================
+ Hits         400     542     +142     
+ Misses      5252    5135     -117     

Flag	Coverage Δ
gui_units	12.03% <95.55%> (+3.30%)	:arrow_up:
svc_units	2.27% <ø> (ø)

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

Impacted Files	Coverage Δ
src/config.ts	0.00% <0.00%> (ø)
src/networking/WebrtcConnectionToService.ts	100.00% <100.00%> (+100.00%)	:arrow_up:
src/util/randomAlphaString.ts	100.00% <100.00%> (ø)
src/util/sleepMsec.ts	100.00% <100.00%> (ø)

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[magland]

Looks good to me.

[jsoules]

Sounds good. I'll get it merged in shortly (currently working on fallback-to-http and I want to hit a stopping point on that first).