Bring in createStatsCollector from Mocha 6 as simple compatibility fix for Mocha 6 users

[usergenic]

Fixes #3394

(By "bring in" I mean "copy+paste" and add types to the createStatsCollector code from the mocha package.)

[usergenic]

FWIW, I just verified in https://travis-ci.org/Polymer/tools/builds/539457197 that without this fix, we get these errors in the wct-mocha integration tests when using mocha 6:

wct-mocha: > [email protected] test:integration /home/travis/build/Polymer/tools/packages/wct-mocha
wct-mocha: > cd test && npm run test:wct
wct-mocha: > [email protected] test:wct /home/travis/build/Polymer/tools/packages/wct-mocha/test
wct-mocha: > bash run-wct.sh
wct-mocha: Installing and starting Selenium server for local browsers
wct-mocha: Selenium server running on port 37216
wct-mocha: [BABEL] Note: The code generator has deoptimised the styling of undefined as it exceeds the max of 500KB.
wct-mocha: [BABEL] Note: The code generator has deoptimised the styling of undefined as it exceeds the max of 500KB.
wct-mocha: chrome 74                Beginning tests via http://localhost:8081/components/wct-mocha-test/generated-index.html?cli_browser_id=0
wct-mocha: chrome failed to maximize
wct-mocha: [BABEL] Note: The code generator has deoptimised the styling of undefined as it exceeds the max of 500KB.
wct-mocha: firefox 66               Beginning tests via http://localhost:8081/components/wct-mocha-test/generated-index.html?cli_browser_id=1
wct-mocha: [BABEL] Note: The code generator has deoptimised the styling of undefined as it exceeds the max of 500KB.
wct-mocha: chrome 74                Tests failed: Timed out
wct-mocha: firefox 66               Tests failed: Timed out
wct-mocha: Test run ended in failure: Timed out
wct-mocha: chrome 74 (error)                     
wct-mocha: firefox 66 (error)                    
wct-mocha: Error: Timed out

[usergenic]

Verified in https://travis-ci.org/Polymer/tools/builds/539471275 that this fix does not break projects which still use Mocha 5, so this can be a patch version.