checker: skip merge regions that are marked nomerge

[disksing]

Signed-off-by: disksing [email protected]

What problem does this PR solve?

Fix #4001

What is changed and how it works?

When regions are marked nomerge, don't merge them.
Also fixed an unstable test.

Check List

Tests

• Unit test

Release note

Support using region labels to control specific regions from being merged.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• rleungx

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[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: bd9147a

[codecov]

Codecov Report

Merging #4006 (bd9147a) into master (0befeb1) will increase coverage by 0.08%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #4006      +/-   ##
==========================================
+ Coverage   74.50%   74.59%   +0.08%     
==========================================
  Files         254      254              
  Lines       25822    25826       +4     
==========================================
+ Hits        19239    19264      +25     
+ Misses       4864     4849      -15     
+ Partials     1719     1713       -6     

Flag	Coverage Δ
unittests	74.59% <71.42%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/schedule/checker/merge_checker.go	74.72% <71.42%> (-1.14%)	⬇️
pkg/tempurl/tempurl.go	45.00% <0.00%> (-15.00%)	⬇️
pkg/dashboard/adapter/manager.go	78.72% <0.00%> (-4.26%)	⬇️
client/base_client.go	82.29% <0.00%> (-2.09%)	⬇️
pkg/btree/btree.go	85.78% <0.00%> (-1.69%)	⬇️
server/grpc_service.go	47.00% <0.00%> (-0.12%)	⬇️
client/client.go	71.96% <0.00%> (+0.13%)	⬆️
server/tso/allocator_manager.go	67.77% <0.00%> (+0.16%)	⬆️
pkg/mock/mockcluster/mockcluster.go	95.28% <0.00%> (+0.24%)	⬆️
server/cluster/cluster.go	82.58% <0.00%> (+0.35%)	⬆️
... and 8 more

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