Fix bug with resource data offset in FreeSpaceModifier introduced in #505

[alchzh]

• I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!)
Fix bug with resource data offset in free_space.py introduced in #505. (No changelog necessary, fixes a bug recently introduced)

Link to Related Issue(s)

Please describe the changes in your request.

• Fix a typo where (await resource.get_data_range_within_parent()).start was accidentally replaced with (await parent.get_data_range_within_parent()).start in FreeSpaceModifier
• Change the free space modifier tests to use a child memoryregion with non-zero offset to catch this bug in the future.

Anyone you think should look at this, specifically?

[whyitfor]

Thanks for catching this. Please see my suggestions -- they mainly revolve around making this test case a little easier to understand.

While I agree that this doesn't require a separate change log entry, i wonder if we should update the existing one to list both 505 and 509? @rbs-jacob, we should probably have a standard approach to things like this.

[rbs-jacob]

While I agree that this doesn't require a separate change log entry, i wonder if we should update the existing one to list both 505 and 509? @rbs-jacob, we should probably have a standard approach to things like this.

I'm in favor of standardizing on listing all relevant PRs for a changelog entry. I'll make a PR that updates the contributor guidelines accordingly to codify this.

[whyitfor]

@alchzh, per #569 (comment), can you update the CHANGELOG to list this PR in addition to 505?

Other than that this looks good to me, feel free to merge in once the CHANGELOG is updated and you are ready.