[fuchsia] Mark tests group testonly

Bug: fuchsia:22679
Change-Id: Ief6171ef8b00997edfc36e6fb6214c582a1b3f11
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/2315007
Commit-Queue: Mark Mentovai <mark@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>

BUILD.gn

@@ -176,6 +176,8 @@ if (crashpad_is_in_chromium || crashpad_is_in_fuchsia) {
     }
 
     group("tests") {
+      testonly = true
+
       deps = [ ":crashpad_test" ]
     }
   }