test: add failing test for hashSalt in prod mode (realContentHash)

Setting output.hashSalt together with optimization.realContentHash (e.g. prod mode)
results in wrong urls for asset-modules, resulting in broken bundles.

Added test seems to be minimum to reproduce.

test/cases/asset-modules-salted/expected/main.css

@@ -0,0 +1,5 @@
+body {
+  background: red;
+  background-image: url(c9e192c015437a21dea1.svg);
+}
+

test/cases/asset-modules-salted/index.js

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+import './style.css';

test/cases/asset-modules-salted/style.css

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+body {
+  background: red;
+  background-image: url(./react.svg);
+}

test/cases/asset-modules-salted/test.filter.js

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+const webpack = require('webpack');
+
+module.exports = () => webpack.version[0] !== '4';

test/cases/asset-modules-salted/webpack.config.js

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+import Self from '../../../src';
+
+/**
+ * Bug report:
+ * Setting output.hashSalt together with optimization.realContentHash (e.g. prod mode)
+ * results in wrong urls for asset-modules.
+ */
+
+module.exports = {
+  entry: './index.js',
+  output: {
+    // if salt is omitted, the realContentHash value works in css
+    hashSalt: 'i break things',
+  },
+  optimization: {
+    // if this is disabled, filenames match even with hashSalt
+    realContentHash: true,
+  },
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [Self.loader, 'css-loader'],
+      },
+      {
+        test: /\.svg$/,
+        type: 'asset/resource',
+      },
+    ],
+  },
+  plugins: [
+    new Self({
+      filename: '[name].css',
+    }),
+  ],
+};