Automatically fetch mazerunner commands rather than waiting for a button press

[kstenerud]

Goal

Mazerunner was flaking too much when the testing app relied upon a button press to trigger the next command fetch.

Automatically fetch commands instead, like we do in bugsnag-cocoa.

[github-actions]

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[twometresteve]

LGTM, with some nice improvements to the test fixture code structure too.