cat: test output path behvaior when target path does not exist

Authored by ryanmce.

Description

cat: test output path behvaior when target path does not exist

In the next patch, we will change the behavior of the cat command to create
output file parent directories before attempting to write the output file.
This patch documents the current unfortunate behavior.

Differential Revision: https://phab.mercurial-scm.org/D1331

Details