pkg/server/bundle/client.TestManagerOnDemandBundleRefresh flaky under race detector

[zmt]

• Version: c5bc6baee2ead4022e73bdfd8ffe722ca3cbb3ea (HEAD of main)
• Platform: Linux hostname 5.4.129+ #1 SMP Wed Aug 18 19:58:18 PDT 2021 x86_64 GNU/Linux
• Subsystem: server

Running the TestManagerOnDemandBundleRefresh is flaky under the race detector. Fails CI/CD in WIP PR #2705. Reproducible with:

% go test -race -count=1000 -failfast -run=TestManagerOnDemandBundleRefresh ./pkg/server/bundle/client
--- FAIL: TestManagerOnDemandBundleRefresh (0.03s)
    manager_test.go:110:
        	Error Trace:	manager_test.go:110
        	Error:      	Not equal:
        	            	expected: 1
        	            	actual  : 2
        	Test:       	TestManagerOnDemandBundleRefresh
FAIL
FAIL	github.com/spiffe/spire/pkg/server/bundle/client	21.045s
FAIL