Remove long path check in test import process.

In this CL:
 - Remove code in test_copier.py related to long paths
 - Unskip tests that are skipped due to long paths

Context: https://codereview.chromium.org/2708883004/.

BUG=609871

Review-Url: https://codereview.chromium.org/2722843002
Cr-Commit-Position: refs/heads/master@{#453845}
3 files changed