Move table transformation tests to tests/test_table_transforms.py

[jeromekelleher]

A new test file test_table_transforms.py was added in #2240. This is intended to be used for things like trim, etc, that do transformations on the tables.

Move the appropriate tests in here, possibly with some refactoring.

[benjeffery]

Closing for inactivity and labelling "future".

[hyanwong]

Note that most of these functions are tested through their tree sequence equivalents, rather than testing the tables methods directly, so there's not so much that's obvious to move.