fix: fix flaky contention test

[schmidt-sebastian]

Fixes #977

This brings the test in line with https://github.com/schmidt-sebastian/java-firestore/blob/master/google-cloud-firestore/src/test/java/com/google/cloud/firestore/it/ITSystemTest.java#L536

[codecov]

Codecov Report

Merging #979 into master will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #979      +/-   ##
==========================================
- Coverage   97.55%   97.45%   -0.11%     
==========================================
  Files          25       25              
  Lines       15978    15978              
  Branches     1284     1208      -76     
==========================================
- Hits        15588    15572      -16     
- Misses        387      403      +16     
  Partials        3        3              

Impacted Files	Coverage Δ
dev/src/order.ts	98.09% <0.00%> (-1.15%)	⬇️
dev/src/pool.ts	97.75% <0.00%> (-0.90%)	⬇️
dev/src/transaction.ts	96.47% <0.00%> (-0.51%)	⬇️
dev/src/watch.ts	98.77% <0.00%> (-0.37%)	⬇️
dev/src/serializer.ts	98.77% <0.00%> (-0.25%)	⬇️
dev/src/validate.ts	98.05% <0.00%> (-0.25%)	⬇️
dev/src/reference.ts	99.80% <0.00%> (-0.08%)	⬇️
dev/src/index.ts	98.62% <0.00%> (-0.07%)	⬇️

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[thebrianchen]

lgtm