Bugzilla

Running `./mach test testing/web-platform/tests/css/css-multicol/multicol-columns-001.xht` yields 

```
/home/aethanyc/Projects/gecko/obj-firefox/backend.TestManifestBackend is out of date with respect to /home/aethanyc/Projects/gecko/testing/web-platform/mach_commands.py
Test configuration changed. Regenerating backend.
Error running mach:

    ['test', 'testing/web-platform/tests/css/css-multicol/multicol-columns-001.xht']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file test| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

AttributeError: 'NoneType' object has no attribute 'append'

  File "/home/aethanyc/Projects/gecko/testing/mach_commands.py", line 383, in test
    argv=extra_args, test_objects=tests, **kwargs)
  File "/home/aethanyc/Projects/gecko/python/mach/mach/registrar.py", line 152, in dispatch
    return self._run_command_handler(handler, context=context, **kwargs)
  File "/home/aethanyc/Projects/gecko/python/mach/mach/registrar.py", line 109, in _run_command_handler
    result = fn(**kwargs)
  File "/home/aethanyc/Projects/gecko/testing/web-platform/mach_commands.py", line 353, in run_web_platform_tests
    params["include"].append(item["name"])


```

`./mach wpt testing/web-platform/tests/css/css-multicol/multicol-columns-001.xht` is OK.

Running `./mach test testing/web-platform/tests/css/css-multicol/multicol-columns-001.xht` yields 

```
/home/aethanyc/Projects/gecko/obj-firefox/backend.TestManifestBackend is out of date with respect to /home/aethanyc/Projects/gecko/testing/web-platform/mach_commands.py
Test configuration changed. Regenerating backend.
Error running mach:

    ['test', 'testing/web-platform/tests/css/css-multicol/multicol-columns-001.xht']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file test| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

AttributeError: 'NoneType' object has no attribute 'append'

  File "/home/aethanyc/Projects/gecko/testing/mach_commands.py", line 383, in test
    argv=extra_args, test_objects=tests, **kwargs)
  File "/home/aethanyc/Projects/gecko/python/mach/mach/registrar.py", line 152, in dispatch
    return self._run_command_handler(handler, context=context, **kwargs)
  File "/home/aethanyc/Projects/gecko/python/mach/mach/registrar.py", line 109, in _run_command_handler
    result = fn(**kwargs)
  File "/home/aethanyc/Projects/gecko/testing/web-platform/mach_commands.py", line 353, in run_web_platform_tests
    params["include"].append(item["name"])
```

`./mach wpt testing/web-platform/tests/css/css-multicol/multicol-columns-001.xht` is OK.