jitter tests are failing

[nhooyr]

$ go test ./...                                                                                                                                                                             develop
--- FAIL: TestRetry (0.00s)
    --- FAIL: TestRetry/Jitter (0.65s)
    	retry_test.go:143: avg dur: 1.299759ms
    	retry_test.go:146: bad avg 1.299759ms
FAIL
FAIL	github.com/codercom/retry	0.683s