Tests for modified assignment

[sloorush]

Related Issue(s)

• Link or list the issue(s) this PR addresses.
• Part of [Tracking issue] Improve code coverage and tests for op.c #107
• Closes Tests for modified assignment {R}←Xf←Y #133

PR Checklist (Remove options that are not relevant)

• This PR adds tests for a new primitive
• Code coverage has been checked
• Documentation complete (Decision docs, code comments, etc.)

[sloorush]

TODO:

• Nested Arrays
• Decide if selected and indexed assignment should be a part of this or not

[sloorush]

Todo:

• Move all code from modified assignment to a new file than op.c
• Write tests for selective modified assignment and indexed modified assignment
• Check overall code coverage after all of the variations are covered

[sloorush]

• Tests for ∪← and ∩← should be added as well