Use backend specific env vars if present for tests

[sgrif]

Having a single DATABASE_URL is really nice in contexts where you're
likely only using one backend. It makes a lot of sense for Travis, which
runs against one backend at a time. It also makes sense for CLI to
continue to default to it by convention, as it has to look at the URL to
figure out which backend to use.

However, for development on Diesel itself, we are switching between
different backends all the time. This didn't matter as much for SQLite,
since we can just give it some random file path and have it work (and
we're usually connecting to :memory: anyway). For MySQL though, we
need to be more concrete. Where applicable, our test suite will now look
for PG_DATABASE_URL, before falling back to just DATABASE_URL. This
does not apply to our examples, as they assume PG and set the
DATABASE_URL manually in examples/test_all. This also doesn't apply
to CLI's test suite, which sets DATABASE_URL manually as needed, since
it can't run in a transaction and thus needs a different database
per-test.

So developers who frequently run our test suite against more than one
backend will likely have a .env file which looks like:

PG_DATABASE_URL=postgres://localhost/diesel_test
MYSQL_DATABASE_URL=mysql://localhost/diesel_test
SQLITE_DATABASE_URL=/tmp/test.db

[killercup]

Nice. We should probably have a contributing.md documenting stuff like this.

[sgrif]

Agreed. I'll add it to the todo list.