build: do not include material prebuilt theme twice in legacy tests (#16966)

Apparently we include the Angular Material prebuilt theme twice in the
legacy Karma tests. This is because we also generate a second folder
called `esm5` in the package output (for later use in the release
composing step). This folder also contains the prebuilt themes as
a side-effect of copying the assets for esm5 inlining/bundling.

In any case, we should not import the theme using a glob, but rather
use an explicit path (similar to how we do it for the mdc theme).

Author: devversion

test/karma.conf.js

@@ -58,7 +58,7 @@ module.exports = config => {
       // 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',
+        pattern: 'dist/packages/material/core/theming/prebuilt/indigo-pink.css',
         included: true,
         watched: true
       },