This emulates the test matrix that gets run in CI, making it easier to
find failures locally.
There was one conflict in the matrix: when JSON is enabled and TLS is
disabled, two of the doctests would fail. This was previous worked
around as an exclude in the github workflow. I changed the JSON doctest
to use HTTP instead.
By using `RUSTFLAGS="-D dead_code -D unused-variables -D unused"`, we tell
the compiler to upgrade warnings for unused code to errors. This combined
with a more elaborate test matrix means we can have the CI catch feature
flag combos that cause warnings.
