Make database tests run in parallel and not fail because of other failed tests

[dullbananas]

• Create a new build_db_pool_for_tests function which:
  • Builds the pool in a LazyCell which runs migrations and maybe inserts stuff in the initialization function
  • Gets a pooled connection, calls the AsyncConnection::begin_test_transaction method, and returns it by wrapping it in DbPool::Conn
• Remove the serial attribute from tests, since concurrent transactions don't see each other's effects

[dessalines]

Could be faster, but also probably not worth the effort, as the tests don't take that long. Federation tests are much slower anyway.

[dullbananas]

Test failures causing some subsequent tests to fail is a big enough inconvenience in my opinion