Split pdfium_unittests sources.

Move foo_unittest.cpp to the same BUILD.gn file as foo.cpp.
Set up dependencies to make pdfium_unittests pass gn check.

Change-Id: Id9f649d5d0c76fe4254f3887778516abb75fcbce
Reviewed-on: https://pdfium-review.googlesource.com/c/43995
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/.gn b/.gn
index 5f7540a..064cf5d 100644
--- a/.gn
+++ b/.gn
@@ -18,6 +18,7 @@
 
 check_targets = [
   ":pdfium",
+  ":pdfium_unittests",
   "//core/*",
   "//fpdfsdk/*",
   "//fxbarcode/*",