[boringssl] Add "testonly" to BUILD.gn for host builds.

For some reason, this was breaking the escher_linux package.

Change-Id: I873e71d8e0262ad30943463682e03f447483e686
diff --git a/BUILD.gn b/BUILD.gn
index 3301f56..95326fc 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -172,6 +172,7 @@
   }
 } else {
   group("tests") {
+    testonly = true
     deps = [
       ":crypto_test",
       ":ssl_test",