From: Johannes Schindelin <johannes.schindelin@gmx.de>

When grepping through the output of a command in the test suite, there
is always a chance that something goes wrong, in which case there would
not be anything useful to debug.

Let's redirect the output into a file instead, and grep that file, so
that the log can be inspected easily if the grep fails.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Submitted-As: https://lore.kernel.org/git/pull.709.git.git.1581688401866.gitgitgadget@gmail.com