Add tests for radiobutton/checkbox JS properties.

Fix bad indexing discovered as a result.

Change-Id: I6241ee32d11948ed242cf8a0b9f8898e8769a869
Reviewed-on: https://pdfium-review.googlesource.com/c/45470
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

testing/resources/javascript/field.fragment

diff --git a/testing/resources/javascript/field.fragment b/testing/resources/javascript/field.fragment
index f696018..843a813 100644
--- a/testing/resources/javascript/field.fragment
+++ b/testing/resources/javascript/field.fragment
@@ -66,7 +66,8 @@
 >>
 endobj
 {{object 8 0}} <<
-  /FT /Tx
+  /FT /Btn
+  /Ff 32768
   /Parent 5 0 R
   /T (Sub_Z)
   /Type /Annot
@@ -75,7 +76,8 @@
 >>
 endobj
 {{object 9 0}} <<
-  /FT /Tx
+  /FT /Btn
+  /Ff 0
   /Parent 5 0 R
   /T (Sub_B)
   /Type /Annot