Run tests in CI environment

Currently tests only run on X11.64 platform.
But other platforms can be supported with a bit of effort.
Remove default bell sound as it does not play nicely with CI environment
that does not have sound card.

test/.gutconfig.windows.json

@@ -0,0 +1,11 @@
+{
+  "dirs": [
+    "res://platform/windows"
+  ],
+  "prefix": "",
+  "suffix": ".test.gd",
+  "include_subdirs": true,
+  "log_level": 2,
+  "should_exit": true
+}
+}