Create Custom Test Column for Pinned Column in Table

🧹 Tech Debt

PR #2964 introduced pinning fixed width columns. Currently the pinned column tests are using the mapping column. We should create a custom Test column for the main table tests.

original comment:

for these three we don't want to exercise setting pixelWidth directly as that is an internal api. In this case we would be leveraging the mapping column's api so setting width-mode (which indirectly sets pixelWidth).

we can assert the mapping column is behaving as expected by asserting on columnInternals.pixelWidth (in case mapping column changes behavior in the future) but we should not exercise manipulating a private api is achieving the validation result we want.

the ideal scenario is that we have a custom Test column for the core table feature that exercises the pinned column feature standalone of other column types and that the mapping column has it's own separate tests to exercise it's participation as a column opting in to the mapping feature. We don't need to make that change, but something to consider in the future.

Review PR #2964 and the main table tests to see how the mapping column currently supports pinned-column coverage. Add the requested custom Test column and move the relevant pinned-column tests to use it, keeping mapping-column behavior covered separately. Done means the main table tests exercise pinning without depending on the mapping column or its internal API.