Fix options.moduleNameMapper override order with preset (jestjs#3565)

Adjusted the order so that the options mappings are checked first and then the preset mappings are checked.
The preset mappings can be overridden by the options mappings.

packages/jest-config/src/__tests__/normalize.test.js

@@ -827,14 +827,32 @@ describe('preset', () => {
       {},
     );
 
-    expect(options.moduleNameMapper).toEqual([['b', 'b'], ['a', 'a']]);
+    expect(options.moduleNameMapper).toEqual([['a', 'a'], ['b', 'b']]);
     expect(options.modulePathIgnorePatterns).toEqual(['b', 'a']);
     expect(options.setupFiles.sort()).toEqual([
       '/node_modules/a',
       '/node_modules/b',
       '/node_modules/regenerator-runtime/runtime',
     ]);
   });
+
+  test('merges with options and moduleNameMapper preset is overridden by options', () => {
+    const {options} = normalize(
+      {
+        moduleNameMapper: { e: 'ee', c: 'cc', b: 'bb', a: 'aa' },
+        preset: 'react-native',
+        rootDir: '/root/path/foo',
+      },
+      {},
+    );
+
+    expect(options.moduleNameMapper).toEqual([
+      ['e', 'ee'],
+      ['c', 'cc'],
+      ['b', 'bb'],
+      ['a', 'aa'],
+    ]);
+  });
 });
 
 describe('preset without setupFiles', () => {

packages/jest-config/src/normalize.js

@@ -76,6 +76,7 @@ const setupPreset = (
   if (options.moduleNameMapper && preset.moduleNameMapper) {
     options.moduleNameMapper = Object.assign(
       {},
+      options.moduleNameMapper,
       preset.moduleNameMapper,
       options.moduleNameMapper,
     );