[analysis_server] Remove useLineEndingsForPlatform=false from imported_elements_computer_test

+ migrate to TestCode instead of searching for substrings.

Change-Id: I4ace282e73eaa140733d35353ea62147e8851f1a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/444021
Reviewed-by: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: DanTup