go/client: make test more resilient (#1361)
go/client: make test more resilient (#1361)

Only fail the retry timing test if the difference in timings is
both:
 - more than 100ms out
 - more than 10% out.

Hopefully this will reduce test flakes.
daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,google/certificate-transparency-go