[io] Fix a test failure when there are multiple linked paths to tmp

The only semantic change is to `testAbsolute`

Change-Id: Ie6f5ed3265163176e0a5f16ca23f7da5f058fe75
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/420520
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Brian Quinlan <[email protected]>

Author: brianquinlan