[graceful] Flaky unit tests

[brandon-welsch]

Execute the tests in module ./graceful
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--- FAIL: TestService_Shutdown_WithRequest (0.63s)
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    --- FAIL: TestService_Shutdown_WithRequest/signal_interrupt_expect_service_to_stop (0.23s)
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        service_test.go:95: 
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            	Error Trace:	/home/rof/src/github.com/Scalingo/go-utils/graceful/service_test.go:95
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            	Error:      	Received unexpected error:
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            	            	Get "http://localhost:9000/?sleep=200": dial tcp [::1]:9000: connect: connection refused
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            	Test:       	TestService_Shutdown_WithRequest/signal_interrupt_expect_service_to_stop
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        service_test.go:464: 
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            	Error Trace:	/home/rof/src/github.com/Scalingo/go-utils/graceful/service_test.go:495
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            	            				/home/rof/src/github.com/Scalingo/go-utils/graceful/service_test.go:464
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            	            				/home/rof/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.0.linux-amd64/src/runtime/asm_amd64.s:1700
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            	Error:      	Received unexpected error:
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            	            	os: process already finished
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            	Test:       	TestService_Shutdown_WithRequest/signal_interrupt_expect_service_to_stop
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            	Messages:   	process 49631 is dead after 100ms
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FAIL
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FAIL	github.com/Scalingo/go-utils/graceful	8.635s
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FAIL