tests: Pass amount to Template.render as string.

render is required to convert the string to Decimal for historical design
reasons. Passing the amount as a string verifies this behavior.

tests/test_hook_ledger_entry.py

@@ -23,7 +23,7 @@ def template_from(section_name, *args, **kwargs):
 
 def template_vars(payee, amount, currency='USD', date=DATE, other_vars=None):
     call_vars = {
-        'amount': decimal.Decimal(amount),
+        'amount': amount,
         'currency': currency,
         'date': date,
         'payee': payee,