Spell out element access explicitly in test code

...to avoid the upcoming loplugin:data asking to replace just each initial
&aAttrs[0] with aAttrs.begin() and leave the remaining &aAttrs[N] alone, which
would make the results look odd.

Change-Id: I7d5c76e9d4fc6c7fa13cb28fca5ea65ea2d0f77e
Reviewed-on: https://gerrit.libreoffice.org/72764
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>