sync: Update `tests.toml` if the corresponding `canonical-data.json` alters a test's `description`

### Example

1. Say that a track has implemented an exercise `bob`, which has an up-to-date `tests.toml` file.
2. Change the `description` of a test in the `canonical-data.json` of that exercise in `exercism/problem-specifications` (assuming that we decide that the `description` is mutable).
3. Run `configlet sync -u --tests`

### Current behaviour

The `tests.toml` file is not changed.

### Expected behaviour

The `tests.toml` file should contain the new `description`

### Context

See https://github.com/exercism/problem-specifications/pull/1687#issuecomment-707850678.

@ee7 wrote:
> Should it update the `tests.toml` file to overwrite the comment that contains the description?"
I think the answer is yes, but it currently doesn't.

@iHiD wrote:
> If we went down the mutable route, then yes it should.