Fixed #35967 -- Deferred test suite fixtures serialization after all dbs setup.

While the top-level objects fed to serialization are bound to the test
database being created nothing prevents code invoked during
serialization from performing queries against other connections entries
that haven't been swapped yet.

The reported example of that is a database router directing all reads to
a test mirror for a set of models involving auto-created many-to-many
fields. It might be tempting to address the many-to-many field case but
this a symptom of a larger problem where the test framework yields the
flow execution to user code that could interact with non-test databases
in unexpected ways.

Deferring test database fixture serialization until the point where all
connections entries have been swapped for their test equivalent ensures
that no code triggered during serialization can interact with non-test
databases.

Thanks Jake Howard for the report and Jacob Walls for the initial
investigation.

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