test: fix travis ci (#2025)

[sre-bot]

cherry-pick #2025 to release-3.1

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Signed-off-by: nolouch nolouch@gmail.com

What problem does this PR solve?

go mod tidy failed.

What is changed and how it works?

lazy install goveralls.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2029 into release-3.1 will increase coverage by 0.01%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           release-3.1    #2029      +/-   ##
===============================================
+ Coverage        69.51%   69.52%   +0.01%     
===============================================
  Files              171      171              
  Lines            16814    16814              
===============================================
+ Hits             11688    11690       +2     
+ Misses            4148     4146       -2     
  Partials           978      978

Impacted Files	Coverage Δ
server/util.go	70.7% <0%> (-4.46%)	⬇️
server/region_syncer/client.go	78.94% <0%> (-3.95%)	⬇️
server/namespace_cluster.go	81.81% <0%> (-3.9%)	⬇️
server/tso.go	74.35% <0%> (-1.71%)	⬇️
client/client.go	74.04% <0%> (+0.63%)	⬆️
server/grpc_service.go	60.55% <0%> (+0.63%)	⬆️
server/leader.go	83.26% <0%> (+1.22%)	⬆️
server/schedulers/shuffle_hot_region.go	65.38% <0%> (+6.41%)	⬆️
pkg/tempurl/tempurl.go	71.42% <0%> (+14.28%)	⬆️

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[lhy1024]

/run-all-tests