Add the "testonly" flag to the GN test target.

build/config/android/rules.gni

Index: build/config/android/rules.gni
diff --git a/build/config/android/rules.gni b/build/config/android/rules.gni
index a982f91b0236ff3e35113866d9bc1de6cdfa2c47..a5da3d0a5809d0b211b055b4fb4444f9140346a5 100644
--- a/build/config/android/rules.gni
+++ b/build/config/android/rules.gni
@@ -575,6 +575,7 @@ template("android_java_prebuilt") {
 #   native_libs: List paths of native libraries to include in this apk. If these
 #     libraries depend on other shared_library targets, those dependencies will
 #     also be included in the apk.
+#   testonly: Marks this target as "test-only".
 #
 #   DEPRECATED_java_in_dir: Directory containing java files. All .java files in
 #     this directory will be included in the library. This is only supported to
@@ -647,6 +648,9 @@ template("android_apk") {
     if (defined(invoker.deps)) {
       deps = invoker.deps
     }
+    if (defined(invoker.testonly)) {
+      testonly = invoker.testonly
+    }
 
     native_libs = _native_libs
   }
@@ -801,6 +805,7 @@ template("unittest_apk") {
     if (defined(invoker.deps)) {
       deps = invoker.deps
     }
+    testonly = true
   }
 }