Add a unit_test_support target.

Split out XFA-only unit test code into xfa_unit_test_support.h. Add a
target for it since it is for unit tests only but there is no other
place to put it.

Change-Id: I0a6e291a4827b754236ddd8040528c0e7489d3a4
Reviewed-on: https://pdfium-review.googlesource.com/c/49912
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 52c22d6..afe61c0 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -252,10 +252,10 @@
     "core/fxcodec:unittests",
     "core/fxcrt",
     "core/fxcrt:unittests",
-    "core/fxge",
     "core/fxge:unittests",
     "fpdfsdk:unittests",
     "testing:test_support",
+    "testing:unit_test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
@@ -277,7 +277,6 @@
     deps += [
       "fxbarcode:unittests",
       "xfa/fde:unittests",
-      "xfa/fgas",
       "xfa/fgas:unittests",
       "xfa/fgas/layout:unittests",
       "xfa/fwl:unittests",