Add testonly field to Antlion build

This is required for Antlion to use Honeydew library as a dependency.

Change-Id: I2aa756531c084fc62049a53d4059dc63f62917fb
Reviewed-on: https://fuchsia-review.googlesource.com/c/antlion/+/921077
Reviewed-by: Sam Balana <sbalana@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Fuchsia-Auto-Submit: Patrick Lu <patricklu@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 653902c..865db6a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -41,6 +41,7 @@
 
 python_library("antlion") {
   source_root = "//third_party/antlion/packages/antlion"
+  testonly = true
   sources = [
     "__init__.py",
     "base_test.py",