Add support for check constraints on MariaDB 10.2+.

[Mariusz Felisiak]

MariaDB 10.2+ support check constraints (see ​check-constraints), we should support them because we officially support this database. I created ​PR to fix feature flags but it requires more work e.g. with introspection.

10 tests fail at 698df6a009cb1c4dbd55905264f24f6edf41066e:

• constraints.tests.CheckConstraintTests.test_name
• constraints.tests.CheckConstraintTests.test_database_constraint
• introspection.tests.IntrospectionTests.test_get_constraints
• migrations.test_operations.OperationTests.test_add_constraint
• migrations.test_operations.OperationTests.test_add_constraint_percent_escaping
• migrations.test_operations.OperationTests.test_add_or_constraint
• migrations.test_operations.OperationTests.test_create_model_with_constraint
• migrations.test_operations.OperationTests.test_remove_constraint
• schema.tests.SchemaTests.test_check_constraints
• schema.tests.SchemaTests.test_remove_field_check_does_not_remove_meta_constraints

[1] ​https://mariadb.com/kb/en/library/information-schema-check_constraints-table/

[Hasan Ramezani]

3 tests fail:

• introspection.tests.IntrospectionTests.test_get_constraints
• schema.tests.SchemaTests.test_check_constraints
• schema.tests.SchemaTests.test_remove_field_check_does_not_remove_meta_constraints

The information_schema.key_column_usage table, does not contain CHECK constraints records. we can find them in information_schema.table_constraints table.
The problem is that records in information_schema.table_constraints do not have column_name and we have to add them to constraints with an empty list of columns ​https://github.com/django/django/pull/11566/files#diff-ed5c74bb534df74e38bc62e79c84fa12R194.

Those 3 tests fail because of the above-mentioned problem.
Any idea?

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1fc2c70f:

Fixed #30593 -- Added support for check constraints on MariaDB 10.2+.

[Mariusz Felisiak <felisiak.mariusz@…>]

In b2aad9ad:

Refs #30593 -- Added _parse_constraint_columns() hook to introspection on MariaDB.

[Mariusz Felisiak <felisiak.mariusz@…>]

In e3fc9af:

Refs #30593 -- Fixed introspection of check constraints columns on MariaDB.

[Mariusz Felisiak <felisiak.mariusz@…>]

In ffff17d4:

Fixed #34553 -- Fixed improper % escaping of literal in constraints.

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.