When sequences are mixed in array indexing, af::backend::index is called. This pull request adds a test for this function call for the case where the sequences have a step that is not 1.

Description

Additional information about the PR answering following questions:

• Is this a new feature or a bug fix?: Adds a test
• Why these changes are necessary: Tests uncovered case
• Potential impact on specific hardware, software or backends: None
• New functions and their functionality: None
• Can this PR be backported to older versions?: Yes
• Future changes not implemented in this PR.: None

Tests pull request #3585

Changes to Users

• Additional options added to the build: None
• No action required by the user

Checklist

• Rebased on latest master
• Code compiles
• Tests pass
• Functions added to unified API
• Functions documented