Use indoc for migrator test fixtures (#64093)

## Objective

Follow-up to #62818.

## Solution

Completed the [fixture cleanup deferred from the original
PR](https://github.com/zed-industries/zed/pull/62818#pullrequestreview-4973821021),
removing `unindent` from the migrator crate’s dependencies.

## Testing

All original migrator tests pass.

## Self-Review Checklist:

- [x] I’ve reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed’s UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- N/A

Cargo.lock

@@ -10961,7 +10961,6 @@ dependencies = [
  "streaming-iterator",
  "tree-sitter",
  "tree-sitter-json",
- "unindent",
 ]
 
 [[package]]

crates/migrator/Cargo.toml

@@ -28,4 +28,3 @@ settings_json = { workspace = true, features = ["editing"] }
 [dev-dependencies]
 indoc.workspace = true
 pretty_assertions.workspace = true
-unindent.workspace = true