jsonclient: increase backoff test leeway (#294)
jsonclient: increase backoff test leeway (#294)

There has been an occasional flake like:
  backoff_test.go:114: backoff.set(30m0s)=59m59.999970667s; want 1h0m0s
google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go,RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go