Replace shell test user

What does this MR do?

it makes sure that we also change the user in the test environment

Why is this MR needed?

If we dont change this, people will run into errors when running the tests:

git clone 'https://gitlab.com/gitlab-org/gitlab-shell.git' 'tmp/tests/gitlab-shell/'
Cloning into 'tmp/tests/gitlab-shell'...
remote: Counting objects: 1332, done.
remote: Compressing objects: 100% (879/879), done.
remote: Total 1332 (delta 792), reused 717 (delta 400)
Receiving objects: 100% (1332/1332), 190.34 KiB | 0 bytes/s, done.
Resolving deltas: 100% (792/792), done.
Checking connectivity... done.
git fetch origin && git reset --hard $(git describe v1.9.6 || git describe origin/v1.9.6)
bin/install
mkdir: /home/git/.ssh: Operation not supported
bin/install failed
rake aborted!
Command failed with status (1): [bin/install...]
/Users/jeroen/Development/gitlab/gitlab/lib/tasks/gitlab/shell.rake:49:in `block (4 levels) in <top (required)>'
/Users/jeroen/Development/gitlab/gitlab/lib/tasks/gitlab/shell.rake:24:in `chdir'
/Users/jeroen/Development/gitlab/gitlab/lib/tasks/gitlab/shell.rake:24:in `block (3 levels) in <top (required)>'
Tasks: TOP => gitlab:shell:install
(See full trace by running task with --trace)

Belongs to what issues? #8 (closed)