Merge to XFA: Consolidate test support code.

Original Review URL: https://codereview.chromium.org/1416713002 .
(cherry picked from commit 6d34d056a7188b7ae9800fe3ceb68645e0442879)

Conflicts:
  BUILD.gn
  samples/BUILD.gn
  samples/pdfium_test.cc
  samples/samples.gyp

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1413593003 .
8 files changed