[1.7.x] Appeased flake8.

Backport of b1432bfc from master
[1.7.x] Reorganized backends tests.

This reduces the number of explicit vendor checks.

Backport of d6672631 from master
[1.7.x] Simplified indexes tests.

Backport of 43a80f48 from master
[1.7.x] Replaced vendor checks by three feature flags.

Backport of c70a61eb from master
[1.7.x] Split ignores_nulls_in_unique_constraints feature.

Oracle and SQL Server don't have exactly the same limitations. It's
worth treating them differently.

Backport of cff59bed from master
[1.7.x] Added feature flags for introspection capabilities.

Backport of 99d9fa32 from master