Improvements to mempool_updatefromblock.py

Summary:
The test creates a mempool with a large number of chained transactions, mining them in batch to avoid crossing the limits.

However the comment explains that it avoids the ancestor limit, which is no longer true since D12589.

This diff:

• Updates the obsolete comment
• Makes the descendant count limit explicit rather than relying on the default
• Make the intent obvious by avoiding magic numbers

There is no change in behavior.

Test Plan:

./test/functional/test_runner.py mempool_updatefromblock

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13131