Add a test case for resolve.fallback

test/configCases/resolve/fallback/a/1.js

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+module.exports = 1;

test/configCases/resolve/fallback/a/2.js

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+module.exports = 'not 2';

test/configCases/resolve/fallback/b/2.js

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+module.exports = 2;

test/configCases/resolve/fallback/index.js

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+it("ignores the fallback if an existing module is present", () => {
+	const two = require("./b/2");
+	expect(two).toBe(2);
+});
+
+it("can fallback if the module does not exist", () => {
+	const one = require("./b/1");
+	expect(one).toBe(1);
+});

test/configCases/resolve/fallback/webpack.config.js

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+const path = require("path");
+/** @type {import("../../../../").Configuration} */
+module.exports = {
+	resolve: {
+		fallback: {
+			"./b": path.resolve(__dirname, "a")
+		}
+	}
+};