TableStructureSpec: use right var

[pjonsson]

What this PR does

The other tests create a TableStructure
object and then calls loadFromJson on
that object, so do the same in the last
test.

Test me

Test only changes.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[na9da]

The file test/Map/TableStructureSpec.js and test/Map/TableColumnSpec.js are stale. It might be better to remove them instead.

[pjonsson]

@na9da alright, fixed!

Thanks for getting the merge-train going again. It would be nice to get #7218 merged so an upper bound for mobx version is added. And also ESLint 8 (#7150) since it shouldn't affect TerriaJS itself, but brings you one major version closer to the latest version.

[na9da]

Thanks @pjonsson!