Schema Inference Tests

[killercup]

There are several places where the schema inference is tested, e.g., in the integration tests of diesel_tests, or the CLI tests for diesel print-schema. I think it'd make sense to also add some tests that focus only on creating a table and asserting the correct table! call gets written.

To implement this, you'd probably start by splitting off parts of the expand_infer_table_from_schema function into a new fn that accepts a given connection. Then, write tests that create a table, call the new fn to get the tokens of a table! call, and compare this to an expected output. (Afterwards, make sure the new table has been dropped; this happens automatically when using begin_test_transaction with SQLite and PG but doesn't work with MySQL.)

Interesting edge cases: Unusual type combinations (e.g., arrays of network addresses), multiple primary keys (expect error on >4 with current code), huge amounts of columns.