[analysis_server] Change highlights tests to use line:col instead of offsets

This allows them to run with normalized line endings.

See #60234

Change-Id: Ic569407a29a1b6f9c2add20c28c6070cff544bf9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/454020
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: DanTup