Update conformance tests to fit the new format

[IlyaFaer]

Bring Storage conformance tests exactly to v4_signatures.json format (see comment). This should be done after merging POST policies PR.

/cc @frankyn, @crwilcox

[IlyaFaer]

@frankyn, in signingV4Tests we have expiration as str:
https://github.com/googleapis/conformance-tests/blob/24a5e2373d1240ff3538f9999888053ba8b6a539/storage/v1/v4_signatures.json#L8

But in postPolicyV4Tests it is int:
https://github.com/googleapis/conformance-tests/blob/24a5e2373d1240ff3538f9999888053ba8b6a539/storage/v1/v4_signatures.json#L281

Should we bring this to a single type?

[frankyn]

I just saw this question.

Yes, bring it in as an int. I'm surprised we didnt' catch that during the signed URLs or Post Policies.