test: coverage for wallet birth time interaction with -reindex

Verifying the wallet updates the birth time accordingly when it
detects a transaction with a time older than the oldest descriptor
timestamp.
This could happen when the user blindly imports a descriptor with
'timestamp=now'.

test/functional/test_runner.py

@@ -206,6 +206,8 @@ BASE_SCRIPTS = [
     'wallet_createwallet.py --descriptors',
     'wallet_watchonly.py --legacy-wallet',
     'wallet_watchonly.py --usecli --legacy-wallet',
+    'wallet_reindex.py --legacy-wallet',
+    'wallet_reindex.py --descriptors',
     'wallet_reorgsrestore.py',
     'wallet_conflicts.py --legacy-wallet',
     'wallet_conflicts.py --descriptors',