Replace merge.smart with a simple merge.

It's not a direct replacement, however this may cover all the needs. If
needed, we can work on the merge strategy.

Jest has a file system cache that is not cleared, not even using
`jest.resetModules()`. As a workaround, we create the webpack config
files with a unique suffix.

Ref:
jestjs/jest#11426 (comment)

lib/build.js

@@ -3,7 +3,7 @@ var fs = require('fs');
 var path = require('path');
 var conf = require('./config');
 var webpack = require('webpack');
-var merge = require('webpack-merge');
+const { merge } = require('webpack-merge');
 const findUp = require('find-up');
 
 const readdir = util.promisify(fs.readdir);
@@ -198,7 +198,7 @@ async function webpackConfig(
   if (userWebpackConfig) {
     var webpackAdditional = require(path.join(cwd, userWebpackConfig));
 
-    return merge.smart(webpackConfig, webpackAdditional);
+    return merge(webpackConfig, webpackAdditional);
   }
 
   return webpackConfig;

lib/build.spec.js

@@ -105,28 +105,60 @@ describe('build', () => {
       ).rejects.toThrow('Cannot find module');
     });
 
-    it('should merge webpack custom config', async () => {
-      const script = `module.exports = () => console.log("hello world")`;
-      await writeFileInFunctions(script, 'index.js');
+    describe('webpack custom config merge', () => {
+      beforeEach(() => jest.resetModules());
+
+      it('should merge plugins', async () => {
+        const script = `module.exports = () => console.log("hello world")`;
+        await writeFileInFunctions(script, 'index.js');
+
+        const webpackConfig = `
+        const webpack = require('webpack');
+        module.exports = { plugins: [new webpack.EnvironmentPlugin(['NODE_ENV'])] };
+        `;
+
+        const suffix = expect.getState().currentTestName.replace(/\s+/g, '_')
+        const userWebpackConfig = await writeFileInBuild(
+          webpackConfig,
+          `webpack/webpack_${suffix}.js`,
+        );
+
+        const stats = await build.run(functions, {
+          userWebpackConfig,
+        });
+        expect(stats.compilation.errors).toHaveLength(0);
+        expect(stats.compilation.options.plugins).toHaveLength(3);
+        expect(
+          stats.compilation.options.plugins.map(
+            (plugin) => plugin.constructor.name,
+          ),
+        ).toEqual(['IgnorePlugin', 'DefinePlugin', 'EnvironmentPlugin']);
+      });
 
-      const webpackConfig = `module.exports = { resolve: { extensions: ['.custom'] } }`;
-      const userWebpackConfig = await writeFileInBuild(
-        webpackConfig,
-        'webpack/webpack.js',
-      );
+      it('should merge resolve extensions', async () => {
+        const script = `module.exports = () => console.log("hello world")`;
+        await writeFileInFunctions(script, 'index.js');
 
-      const stats = await build.run(functions, {
-        userWebpackConfig,
+        const webpackConfig = `module.exports = { resolve: { extensions: ['.custom'] } }`;
+        const suffix = expect.getState().currentTestName.replace(/\s+/g, '_')
+        const userWebpackConfig = await writeFileInBuild(
+          webpackConfig,
+          `webpack/webpack_${suffix}.js`,
+        );
+
+        const stats = await build.run(functions, {
+          userWebpackConfig,
+        });
+        expect(stats.compilation.errors).toHaveLength(0);
+        expect(stats.compilation.options.resolve.extensions).toEqual([
+          '.wasm',
+          '.mjs',
+          '.js',
+          '.json',
+          '.ts',
+          '.custom',
+        ]);
       });
-      expect(stats.compilation.errors).toHaveLength(0);
-      expect(stats.compilation.options.resolve.extensions).toEqual([
-        '.wasm',
-        '.mjs',
-        '.js',
-        '.json',
-        '.ts',
-        '.custom',
-      ]);
     });
 
     describe('babel config file resolution', () => {