rwbarton authored Jan 10, 2017 and Ben Gamari committed Jan 10, 2017

If you happen to have a T1234.run.stdout file lying aroud (probably
from before the move to running tests in temporary subdirectories)
it gets symlinked into the T1234.run directory since its name starts
with T1234; and then program output gets appended to the existing
file (through the symlink). We should open the file for writing
instead, to replace the symlink with a new file.

Test Plan: tested locally, + harbormaster

Reviewers: austin, Phyx, bgamari

Reviewed By: Phyx, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2946