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;
}

0 files changed, 0 insertions, 0 deletions