merkle: Cover rehash paths in tests for log_proofs (#2146)
merkle: Cover rehash paths in tests for log_proofs (#2146)

This change improves test coverage for inclusion proof generation in cases
when the requested tree size differs from the stored tree size. These are the
cases when "rehashing" is needed.

This is the most likely query scenario for non-static trees (e.g., for CT a client
probably requests inclusion proofs for an older tree than the current one), yet
until now is wasn't covered by tests at all (maybe only in integration tests).
gdbelvin/trillian,gdbelvin/trillian,AlCutter/trillian,AlCutter/trillian,google/trillian,gdbelvin/trillian,google/trillian