add comments to explain the test rollup and karma config files

actionview/karma.conf.js

@@ -1,3 +1,5 @@
+// Karma configuration for running the UJS tests
+
 const config = {
   browsers: ["ChromeHeadless"],
   frameworks: ["qunit"],

actionview/rollup.config.test.js

@@ -1,3 +1,5 @@
+// Rollup configuration for compiling the UJS tests
+
 import commonjs from "@rollup/plugin-commonjs"
 import resolve from "@rollup/plugin-node-resolve"