Move fontconfig test support into //third_party/test_fonts

//third_party/test_fonts is currently a submodule so that other
projects can pull it in.  However, the test support logic is currently
in //base.  This CL moves the logic into //third_party/test_fonts and
removes //base dependencies.  This should make it easier for projects
like PDFium to use a hermetic font configuration.

R=thestig

Bug: 1250250
Change-Id: If3a3277e0888fcbf32af71a59e174acd202f95bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3203358


Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#928890}