test: fix failure in feature_nulldummy.py on single-core machines

On single-core machines, executing the test feature_nulldummy.py results in
the following assertion error:

...
2021-08-18T15:37:58.805000Z TestFramework (INFO): Test 4: Non-NULLDUMMY base multisig transaction is invalid after activation
2021-08-18T15:37:58.814000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "[...]/test/functional/test_framework/test_framework.py", line 131, in main
    self.run_test()
  File "[...]/test/functional/feature_nulldummy.py", line 107, in run_test
    self.block_submit(self.nodes[0], [test4tx], accept=False)
  File "[...]/test/functional/feature_nulldummy.py", line 134, in block_submit
    assert_equal(None if accept else 'block-validation-failed', node.submitblock(block.serialize().hex()))
  File "[...]/test/functional/test_framework/util.py", line 49, in assert_equal
    raise AssertionError("not(%s)" % " == ".join(str(arg) for arg in (thing1, thing2) + args))
AssertionError: not(block-validation-failed == non-mandatory-script-verify-flag (Dummy CHECKMULTISIG argument must be zero))
2021-08-18T15:37:58.866000Z TestFramework (INFO): Stopping nodes
...

The behaviour can be reproduced on a multi-core machine by simply changing the
function GetNumCores() (in src/util/system.cpp) to return 1:

int GetNumCores()
{
    return 1;
}

test/functional/feature_nulldummy.py

@@ -54,6 +54,7 @@ class NULLDUMMYTest(BitcoinTestFramework):
         self.extra_args = [[
             f'-segwitheight={COINBASE_MATURITY + 5}',
             '-addresstype=legacy',
+            '-par=1',  # Use only one script thread to get the exact reject reason for testing
         ]]
 
     def skip_test_if_missing_module(self):
@@ -131,7 +132,7 @@ class NULLDUMMYTest(BitcoinTestFramework):
             add_witness_commitment(block)
         block.rehash()
         block.solve()
-        assert_equal(None if accept else 'block-validation-failed', node.submitblock(block.serialize().hex()))
+        assert_equal(None if accept else NULLDUMMY_ERROR, node.submitblock(block.serialize().hex()))
         if accept:
             assert_equal(node.getbestblockhash(), block.hash)
             self.lastblockhash = block.hash