[viola] Don't name the test package "tests".

It conflicts with another package (also being renamed) in Garnet.

TEST=fx build

Change-Id: I1f99e1a6ea75ced9a73a980901ae271d9c83a569
diff --git a/bin/voila/BUILD.gn b/bin/voila/BUILD.gn
index f3a3fe8..d99abc6 100644
--- a/bin/voila/BUILD.gn
+++ b/bin/voila/BUILD.gn
@@ -36,6 +36,7 @@
 
 package("tests") {
   testonly = true
+  package_name = "voila-tests"
   deps = [
     ":bin",
   ]