Create .test_config

Use package:test to test shelf_static.  Test only on vm, since tests use dart:io
diff --git a/.test_config b/.test_config
new file mode 100644
index 0000000..632d153
--- /dev/null
+++ b/.test_config
@@ -0,0 +1,5 @@
+{
+  "test_package": {
+    "platforms": ["vm"]
+  }
+}