Convert library prefix completion tests

Change-Id: I2c433ed634d70bb3e03f343503c4e0052ef4df4d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/302140
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson