Disable Jest's default transformer (Babel) (#10454)

* Disable Jest's default transform (Babel)

https://jestjs.io/docs/en/next/code-transformation#defaults

Otherwise, I sometimes get this error because Babel tries to resolve Browserslist:

----
  ● plugin search should not crash when prettier isn't inside a directory › encountered a declaration exception

    ENOENT: no such file or directory, stat '/mnt/d/dev/prettier/tests_integration/mnt/d/dev/prettier/tests_integration/runPrettier.js'

      52 |   jest.spyOn(fs, "statSync").mockImplementation((filename) => {
      53 |     if (path.basename(filename) === "virtualDirectory") {
    > 54 |       return origStatSync(path.join(__dirname, __filename));
         |              ^
      55 |     }
      56 |     return origStatSync(filename);
      57 |   });

      at Object.<anonymous> (tests_integration/runPrettier.js:54:14)
      at isFile (node_modules/browserslist/node.js:38:42)
      at eachParent (node_modules/browserslist/node.js:46:13)
      at Object.getStat (node_modules/browserslist/node.js:197:15)
      at browserslist (node_modules/browserslist/index.js:433:19)
      at resolveTargets (node_modules/@babel/helper-compilation-targets/lib/index.js:174:46)
----

jest.config.js

@@ -18,7 +18,7 @@ if (INSTALL_PACKAGE || (isProduction && !TEST_STANDALONE)) {
 process.env.PRETTIER_DIR = PRETTIER_DIR;
 
 const testPathIgnorePatterns = [];
-let transform;
+let transform = {};
 if (TEST_STANDALONE) {
   testPathIgnorePatterns.push("<rootDir>/tests_integration/");
 }