[fuchsia] Mark runtime_kernel testonly

It depends on :runtime, which is marked testonly, and so must be
testonly as well.

Change-Id: Ib81860de1de679c89268fa2af5d86a3f2a1c9624
Reviewed-on: https://dart-review.googlesource.com/64082
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Zach Anderson <zra@google.com>

BUILD.gn

@@ -63,6 +63,10 @@ group("runtime") {
 }
 
 group("runtime_kernel") {
+  if (targetting_fuchsia) {
+    # Fuchsia has run_vm_tests marked testonly.
+    testonly = true
+  }
   deps = [
     ":runtime",
   ]