Run dumpwallet for legacy wallets only in wallet_backup.py

Descriptor wallets don't support dumpwallet, so make the tests that do
dumpwallet legacy wallet only.

test/functional/test_runner.py

@@ -88,6 +88,7 @@ BASE_SCRIPTS = [
     'wallet_hd.py',
     'wallet_hd.py --descriptors',
     'wallet_backup.py',
+    'wallet_backup.py --descriptors',
     # vv Tests less than 5m vv
     'mining_getblocktemplate_longpoll.py',
     'feature_maxuploadtarget.py',