[gn] Add a temporary forwarding target to rename "libc-tests" to "nacl-libc-tests"

We want to use the "libc-tests" for first-party tests of our
libc. Since this is outside fuchsia.git, we first add a temporary
forwarding shim.

Change-Id: I0af3f675e9619ba12ba71b9dc783a6750bc84a3f
diff --git a/BUILD.gn b/BUILD.gn
index 52c33f0..930ce34 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -85,3 +85,10 @@
     },
   ]
 }
+
+# This is a temporary forwarding target so that we can rename "libc-tests" to
+# "nacl-libc-tests".
+group("nacl-libc-tests") {
+  testonly = true
+  public_deps = [ ":libc-tests" ]
+}