Fix failing builds on main (btrix-microk8s-test) (#1413)

Because tests aren't meant to be run on the CI builder, testing
dependencies aren't installed. `fork-ts-checker-webpack-plugin` still
was trying to type-check the test files though, and wasn't finding the
types for them. This changes the `fork-ts-checker-webpack-plugin` config
to ignore test files entirely.

Tested by manually in Docker with `scripts/build-frontend.sh`.

Thanks for your help @tw4l!

Fixes build regression introduced in
https://github.com/webrecorder/browsertrix-cloud/pull/1407

frontend/webpack.config.js

@@ -65,6 +65,7 @@ const version = (() => {
   return packageJSON.version;
 })();
 
+/** @type {import('webpack').Configuration} */
 const main = {
   entry: "./src/index.ts",
   output: {
@@ -125,7 +126,9 @@ const main = {
       maxChunks: 12,
     }),
 
-    new ForkTsCheckerWebpackPlugin(),
+    new ForkTsCheckerWebpackPlugin({
+      typescript: { configOverwrite: { exclude: ["**/*.test.ts"] } },
+    }),
 
     new HtmlWebpackPlugin({
       template: "src/index.ejs",