testing: add missing tests for core.py exceptions

[superlopuh]

We're missing some test coverage for IR manipulation exceptions, it would be great to add them for:

• Cannot add region that is already attached on an operation.
• Erased SSA value is used by the operation
• Cannot detach a toplevel operation
• Unexpected index
• Attempting to delete an argument of the wrong block
• Operation is not a child of the block.
• Cannot detach operation from a different block.
• Parent pointer of operation does not refer to containing region
• Parent pointer of block does not refer to containing region

[superlopuh]

Something like this would be a great replacement for "Unexpected index":

raise ValueError(
    f"Cannot insert block argument at index {index}, index must be in "
    f"range [0,{len(self._args)}]."
)