Flaky system tests

[ericywl]

There are 2 system tests that are flaky currently.
Note: I could not reproduce it on local environment, but it happens a lot in CI.

=== RUN   Test_warmup/4_agent_2s
    main_test.go:95: 
        	Error Trace:	/home/runner/work/apm-server/apm-server/systemtest/benchtest/main_test.go:95
        	Error:      	"0" is not greater than "4"
        	Test:       	Test_warmup/4_agent_2s

=== RUN   TestOTLPRateLimit
    otlp_test.go:479: 
        	Error Trace:	/home/runner/work/apm-server/apm-server/systemtest/otlp_test.go:479
        	Error:      	Not equal: 
        	            	expected: 0x8
        	            	actual  : 0x4
        	Test:       	TestOTLPRateLimit
    otlp_test.go:480: 
        	Error Trace:	/home/runner/work/apm-server/apm-server/systemtest/otlp_test.go:480
        	Error:      	Not equal: 
        	            	expected: "traces export: rpc error: code = ResourceExhausted desc = rate limit exceeded"
        	            	actual  : "traces export: rpc error: code = DeadlineExceeded desc = received context error while waiting for new LB policy update: context deadline exceeded"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-traces export: rpc error: code = ResourceExhausted desc = rate limit exceeded
        	            	+traces export: rpc error: code = DeadlineExceeded desc = received context error while waiting for new LB policy update: context deadline exceeded
        	Test:       	TestOTLPRateLimit