fix(developer): kmc code generation for context(n) in context

Fixes #9930.

* Fixes the index offset calculation, with +1 for >=v10.0 keyboards and
  -1 for <v10.0 keyboards (difference is because older version keyboards
  have context offsets calculated in the opposite direction). Note that
  this happened because the `CODE_CONTEXTEX` index is stored in memory
  as 0-based in kmc, but was 1-based in kmcomp.

* Adds unit tests for v9.0 and v10.0 keyboards for `context(n)` in
  context.

* Adds unit tests for v9.0 and v10.0 keyboards for `context(n)` in
  output. These were already correct but now we can be assured that
  they are being tested.

* Enables code coverage for kmw-compiler, and sets a lower threshold for
  pass because coverage is still just over 70%.