Thanks David Sanders for the report.

Special thanks to Hannes Ljungberg for finding multiple implementation
gaps.

Thanks also to Simon Charette, Adam Johnson, and Mariusz Felisiak for
reviews.

Proper escaping of % in string literals used when defining constaints
was attempted (a8b3f96f6) by overriding quote_value of Postgres and
Oracle schema editor. The same approach was used when adding support for
constraints to the MySQL/MariaDB backend (1fc2c70).

Later on it was discovered that this approach was not appropriate and
that a preferable one was to pass params=None when executing the
constraint creation DDL to avoid any form of interpolation in the first
place (42e8cf47).

When the second patch was applied the corrective of the first were not
removed which caused % literals to be unnecessary doubled. This flew
under the radar because the existings test were crafted in a way that
consecutive %% didn't catch regressions.

This commit introduces an extra test for __exact lookups which
highlights more adequately % doubling problems but also adjust a
previous __endswith test to cover % doubling problems (%\% -> %%\%%).

Thanks Thomas Kolar for the report.

Refs #32369, #30408, #30593.

This ensures that constraint violations are tested in isolation from
each other as an IntegrityError only ensures a least one constraint is
violated.

For example, the assertion added in 42e8cf4 break both the
name_constraint_rhs and the rebate_constraint constraints and thus
doesn't constitute a proper regression test. Refs #32369.

This makes squashing migrations an available path for changing
Meta.index_together, which is deprecated, to Meta.indexes.

Follow up to f81032572107846922745b68d5b7191058fdd5f5.

Thanks Simon Charette and David Wobrock for reviews.