test(storage): dual trans for HMAC / PostPolicyV4 (googleapis#6717)

Add dual transport tests for HMAC keys and for PostPolicyV4.

One PostPolicyV4 test creates a second client with custom key behavior. I split this test into two separate tests, one of which fails for gRPC. I left this as a skip for now since it will require some digging in the constructor creds logic to fix.

I also cleaned up some layers of indirection around the various testConfig client methods, and now the multiTransportTest function can take client options to pass down to the client constructors which should make it easier to clean up some other tests which have custom client constructor logic.