Add unit tests for webrtc.js

[vvmnnnkv]

This for #48

Some corner cases need to be covered, however, I've got concern that mocking browser's RTC might be wrong direction, as this doesn't actually test that RTC works. Tests are not being executed in browser(s).
E.g. I tried to run "with-grid" example to establish connection between Chrome and FF, and this didn't work. Maybe this should be covered by some separate integration test suite, though, not sure.

[codecov]

Codecov Report

Merging #52 into grid-syft will increase coverage by 69.71%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           grid-syft    #52       +/-   ##
============================================
+ Coverage      30.28%   100%   +69.71%     
============================================
  Files              2      3        +1     
  Lines            142    165       +23     
  Branches          12     12               
============================================
+ Hits              43    165      +122     
+ Misses            89      0       -89     
+ Partials          10      0       -10

Impacted Files	Coverage Δ
test/_webrtc-mocks.js	100% <100%> (ø)
src/webrtc.js	100% <0%> (+92.52%)	⬆️

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[cereallarceny]

All looks good, a few comments. In the future, should we look at replacing these tests with Puppeteer? https://jestjs.io/docs/en/puppeteer