fix: use rejected Promise for terminate()

[schmidt-sebastian]

This allows us to catch the rejected Promise, rather than having it show up in test log as an unhandled exception.

[codecov]

Codecov Report

Merging #845 into master will increase coverage by 7.97%.
The diff coverage is 55%.

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
+ Coverage    87.4%   95.37%   +7.97%     
==========================================
  Files          27       25       -2     
  Lines       16542    15152    -1390     
  Branches     1151     1151              
==========================================
- Hits        14458    14451       -7     
+ Misses       2078      695    -1383     
  Partials        6        6

Impacted Files	Coverage Δ
dev/src/v1beta1/firestore_client.ts	93.23% <0%> (ø)	⬆️
dev/src/validate.ts	97.42% <100%> (ø)	⬆️
dev/src/pool.ts	96.65% <100%> (+0.01%)	⬆️
dev/src/serializer.ts	95.18% <100%> (ø)	⬆️
dev/src/v1/firestore_admin_client.ts	89.96% <25%> (ø)	⬆️
dev/src/v1/firestore_client.ts	94.7% <66.66%> (-0.04%)	⬇️
types/firestore.d.ts
dev/src/external-modules.d.ts

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[schmidt-sebastian]

Sending this to wilhuff since thebrianchen is still OOOThis PR also includes the fixes + pending feedback for #833