(#425) Use the test font map for the dimensions unit test

Set up the test font map so that the test does not depend on the
system settings. This avoids the test failure for the
"/dimensions/sub/text_position" unit test on ubuntu.
8 jobs for <span class="ref-name">syslang</span> in 36 minutes and 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #225458
fedora test

00:07:23

 
  Lint
manual #225460
allowed to fail manual
clippy
manual #225459
allowed to fail manual
rustfmt
passed #225461
tarball

00:12:18

 
  Cross Distro
passed #225548
debian_i386:testing

00:07:51

passed #225463
debian:testing

00:09:10

passed #225462
fedora:rawhide

00:08:11

failed #225464
debian_i386:testing

00:01:38