Run pdfium_unittests on Linux with hermetic font configuration

* Verified with strace that system fonts are not loaded
* For now, hermetic fonts are only set up on Linux.
  * It's unclear if we want to do this on all platforms.
* Arial/Arimo are metric-compatible, so they are chosen in tests
  * The test font changed from Arial Black to Arial Bold, so some
    test expectations need to be updated for the new font width.

Previous CL:
https://pdfium-review.googlesource.com/c/pdfium/+/86175

Bug: chromium:1250250
Change-Id: I6b8400fd5e76f3b4ac4755ddd5ae8f84a7ca7cd2
R=thestig
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/86370
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/.gitignore b/.gitignore
index 2e2ba13..783e7ca 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,6 +18,7 @@
 /third_party/markupsafe
 /third_party/nasm
 /third_party/skia
+/third_party/test_fonts
 /third_party/zlib
 /tools/clang
 /tools/code_coverage