Make User#username spec deterministic
Make User#username spec deterministic

The previous spec created two Chef OAuth2 accounts, which occassionally
caused spec failures.

chef/supermarket,jzohrab/supermarket,leftathome/supermarket,chef/supermarket,rafaelmagu/supermarket,nellshamrell/supermarket,rafaelmagu/supermarket,robbkidd/supermarket,dpnl87/supermarket,chef/supermarket,jzohrab/supermarket,tas50/supermarket,robbkidd/supermarket,juliandunn/supermarket,tas50/supermarket,leftathome/supermarket,dpnl87/supermarket,jzohrab/supermarket,nellshamrell/supermarket,nellshamrell/supermarket,dpnl87/supermarket,dpnl87/supermarket,robbkidd/supermarket,juliandunn/supermarket,jzohrab/supermarket,leftathome/supermarket,juliandunn/supermarket,tas50/supermarket,rafaelmagu/supermarket,leftathome/supermarket,robbkidd/supermarket,tas50/supermarket,juliandunn/supermarket,nellshamrell/supermarket,rafaelmagu/supermarket,chef/supermarket,chef/supermarket