Merge #12947: Wallet hd functional test speedup and clarification

Summary:
6cba60ace2 speed up wallet_hd.py and clarify/augment checks (Gregory Sanders)

Pull request description:

With `keypool=0` I see no reason to do 300 addresses and sends.

(with --enable-debug)

Before patch:
real	1m10.412s
user	0m49.772s
sys	0m3.988s

After:
real	0m11.566s
user	0m3.344s
sys	0m4.648s

Also added check, since I failed to understand that on startup the wallet already knows about funds by rescanning blocks newer than oldest key birthdate.

Tree-SHA512: cf90f7fe6a437b8b7b1f0707464b9c06085233167826f1a12c3871684664d4d572e13f03e13a718e4537cac39713271c4ac3d9b983e10080b50647caf3cbe82d

Backport of Core PR12947
https://github.com/bitcoin/bitcoin/pull/12947/

Test Plan:

test_runnery.py wallet_hd

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3839