test: move gbt proposal mode tests to new file

Additionally this commit gives each test its
own function.

The assert_submitblock helper is absorbed into
assert_template.

Review hint:
git show --color-moved=dimmed-zebra

test/functional/test_runner.py

@@ -209,6 +209,7 @@ BASE_SCRIPTS = [
     'rpc_decodescript.py',
     'rpc_blockchain.py --v1transport',
     'rpc_blockchain.py --v2transport',
+    'mining_template_verification.py',
     'rpc_deprecated.py',
     'wallet_disable.py',
     'wallet_change_address.py',