This adds tests that complete branch coverage in compact pointer
encoding/decoding code (previously first_attribute was always encoded
using compact encoding in the entire test suite).
This makes sure all .reserve calls failure paths are covered. These
tests don't explicitly test if reserve is present on all paths - this is
much harder to test since not all modifications require reserve to be
called, so we'll have to rely on a combination of automated testing and
sanity checking for this.
Also add more parsing out of memory coverage tests.
