build: add mdc prebuilt theme to karma legacy tests (#16965)

Adds the MDC prebuilt theme to the karma legacy tests. This is
consistent with what we did for the Bazel tests. It can cause
unexpected differences and confusion when debugging tests in
both Bazel and the legacy job (different visual output).

Author: devversion

test/karma.conf.js

@@ -55,12 +55,18 @@ module.exports = config => {
       {pattern: 'test/karma-system-config.js', included: true, watched: false},
       {pattern: 'test/karma-test-shim.js', included: true, watched: false},
 
-      // Include a Material theme in the test suite.
+      // Include a Material theme in the test suite. Also include the MDC theme as
+      // karma runs tests for the MDC prototype components as well.
       {
         pattern: 'dist/packages/**/core/theming/prebuilt/indigo-pink.css',
         included: true,
         watched: true
       },
+      {
+        pattern: 'dist/packages/material-experimental/mdc-theming/prebuilt/indigo-pink.css',
+        included: true,
+        watched: true
+      },
 
       // Includes all package tests and source files into karma. Those files will be watched.
       // This pattern also matches all sourcemap files and TypeScript files for debugging.