test: modernize mocha config

[JustinBeckwith]

No description provided.

[codecov]

Codecov Report

Merging #875 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #875      +/-   ##
==========================================
- Coverage   83.63%   83.59%   -0.04%     
==========================================
  Files          31       31              
  Lines       11390    11390              
  Branches      384      367      -17     
==========================================
- Hits         9526     9522       -4     
- Misses       1863     1867       +4     
  Partials        1        1              

Impacted Files	Coverage Δ
src/message-stream.ts	97.69% <0.00%> (-0.77%)	⬇️
src/subscriber.ts	99.36% <0.00%> (-0.22%)	⬇️

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[gcf-merge-on-green]

Your PR was not mergeable because either one of your required status checks failed, or one of your required reviews was not approved. See required reviews for your repo here: https://github.com/googleapis/sloth/blob/master/required-checks.json

[gcf-merge-on-green]

Your PR was not mergeable because either one of your required status checks failed, or one of your required reviews was not approved. See required reviews for your repo here: https://github.com/googleapis/sloth/blob/master/required-checks.json

[gcf-merge-on-green]

Your PR was not mergeable because either one of your required status checks failed, or one of your required reviews was not approved. See required reviews for your repo here: https://github.com/googleapis/sloth/blob/master/required-checks.json

[feywind]

I think there might actually be an issue here (or uncovered issues from the upgrade). I'll look at this a bit later.

[feywind]

Looks like the same failure here as #875.