Subject: guix-environment-container.sh test fails when building guix on tmpfs

From: Ting-Wei Lan <[email-masked]>
Date: Tue, 24 Nov 2015 23:57:39 +0800

This problem happens on both guix 0.9.0 and master branch.

$tmpdir/mounts can be empty when running tests on tmpfs because
$mount_test_code ignores tmpfs. (`wc -l $tmpdir/mounts` is 0)