migrate feature tests fixtures to a JSON representation

[williballenthin]

it would be convenient for the feature test fixtures to be available as a JSON representation, so that other tools can read the test cases and assert that they also recognize the same features/functions/instructions.

[williballenthin]

while we're at it, we could make the location (like function=0x401000,bb=0x10400A) more structured, like location: {function: 0x401000, bb: 0x40100A}

[williballenthin]

though we'd need to do some research to see how pytest lets you discover test cases from data files. i'm sure it exists but i haven't done this before.

[williballenthin]

https://docs.pytest.org/en/stable/how-to/parametrize.html#pytest-generate-tests