resourcemanager: make TestResourceManagerServer more stable

[HuSharp]

What problem does this PR solve?

Issue Number: Close #8417

What is changed and how does it work?

2024-07-18T08:30:29.5157887Z --- FAIL: TestResourceManagerServer (2.97s)
2024-07-18T08:30:29.5158616Z     server_test.go:62: 
2024-07-18T08:30:29.5160011Z         	Error Trace:	/home/runner/work/pd/pd/tests/integrations/mcs/resourcemanager/server_test.go:62
2024-07-18T08:30:29.5162038Z         	Error:      	Error "rpc error: code = Unavailable desc = not leader" does not contain "resource group not found"
2024-07-18T08:30:29.5163328Z         	Test:       	TestResourceManagerServer
2024-07-18T08:30:29.5163912Z FAIL

checked by

pd/pkg/mcs/resourcemanager/server/grpc_service.go

Lines 89 to 94 in e7c9d15

	func (s *Service) checkServing() error {
	if s.manager == nil || s.manager.srv == nil || !s.manager.srv.IsServing() {
	return errNotLeader
	}
	return nil
	}

Check List

Tests

• Unit test

Release note

None.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.22%. Comparing base (c65577c) to head (1e0f9e4).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8418      +/-   ##
==========================================
- Coverage   77.28%   77.22%   -0.07%     
==========================================
  Files         471      471              
  Lines       61878    61878              
==========================================
- Hits        47823    47783      -40     
- Misses      10470    10501      +31     
- Partials     3585     3594       +9     

Flag	Coverage Δ
unittests	77.22% <ø> (-0.07%)	⬇️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lhy1024, nolouch, okJiang

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-18 09:05:42.951378486 +0000 UTC m=+517564.942319951: ☑️ agreed by lhy1024.
• 2024-07-19 06:54:25.428816468 +0000 UTC m=+596087.419757934: ☑️ agreed by nolouch.