Remove is_valid_now bits of test.
Remove is_valid_now bits of test.

Because:
a) the passage of time should not cause a test to explode
b) the _now() function just delegates to the _at() functions which are
   tested explicitly here anyway.

pphaneuf/certificate-transparency,RJPercival/certificate-transparency,pphaneuf/certificate-transparency,google/certificate-transparency,RJPercival/certificate-transparency,google/certificate-transparency,pphaneuf/certificate-transparency,RJPercival/certificate-transparency,google/certificate-transparency,google/certificate-transparency,pphaneuf/certificate-transparency,RJPercival/certificate-transparency