flaky system-test test TestTailSampling

[carsonip]

Seen in https://github.com/elastic/apm-server/actions/runs/18601289863/job/53040098353?pr=19191

 === RUN   TestTailSampling
    sampling_test.go:158: waiting for 100 "parent" transactions
    sampling_test.go:166: 
        	Error Trace:	/home/runner/work/apm-server/apm-server/systemtest/sampling_test.go:166
        	Error:      	Not equal: 
        	            	expected: 100
        	            	actual  : 200
        	Test:       	TestTailSampling
        	Messages:   	parent
    sampling_test.go:158: waiting for 100 "child" transactions
    sampling_test.go:166: 
        	Error Trace:	/home/runner/work/apm-server/apm-server/systemtest/sampling_test.go:166
        	Error:      	Not equal: 
        	            	expected: 100
        	            	actual  : 200
        	Test:       	TestTailSampling
        	Messages:   	child
    server.go:133: log file: /home/runner/work/apm-server/apm-server/systemtest/logs/TestTailSampling/apm-server
    server.go:133: log file: /home/runner/work/apm-server/apm-server/systemtest/logs/TestTailSampling/apm-server
--- FAIL: TestTailSampling (1.88s)