add test case for shared imports

Author: sokra

test/cases/shared-import/a.css

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+@import "./shared.css";
+
+.shared {
+  color: red;
+}

test/cases/shared-import/b.css

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+@import "./shared.css";
+
+.shared {
+  background-color: red;
+}

test/cases/shared-import/c.css

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+@import "./shared.css";
+
+.shared {
+  border: 1px solid red;
+}

test/cases/shared-import/expected/1.css

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+.shared {
+  border: 1px solid red;
+}
+

test/cases/shared-import/expected/main.css

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+.shared {
+  color: green;
+  background-color: green;
+  border: 1px solid green;
+}
+
+.shared {
+  color: red;
+}
+
+.shared {
+  background-color: red;
+}
+

test/cases/shared-import/index.js

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+import './a.css';
+import './b.css';
+
+import("./c.css");

test/cases/shared-import/shared.css

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+.shared {
+  color: green;
+  background-color: green;
+  border: 1px solid green;
+}

test/cases/shared-import/webpack.config.js

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+const Self = require('../../../');
+
+module.exports = {
+  entry: './index.js',
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [
+          Self.loader,
+          'css-loader',
+        ],
+      },
+    ],
+  },
+  plugins: [
+    new Self({
+      filename: '[name].css',
+    }),
+  ],
+};