Fix non-deterministic test case.

The order of the tuples in a collection is not defined; hence we need to sort
them in test cases that examine collections with > 1 row.
Code cleanup.

Don't explicitly set the `tabname` of the PushElement created for a `pro`
operation; I can't think of a reason why this would be necessary. By avoiding
this, we can make `tabname` a read-only field of `BudCollection`, which is much
more sensible (mutating `tabname` at arbitrary times would be bad).