Chore: Add test for isFakeRoot (yarnpkg#4435)

**Summary**

Follow up to yarnpkg#4431. `isFakeRoot` didn't have any tests and it was broken from the start. yarnpkg#4431 solved it and it was merged to be included in 1.0.2 without tests. This patch adds the missing tests for this function.

**Test plan**

Added new tests, duh :D

__tests__/util/root-user.js

@@ -1,9 +1,24 @@
 /* @flow */
 
-import {isRootUser} from '../../src/util/root-user.js';
+import {isRootUser, isFakeRoot} from '../../src/util/root-user.js';
 
 test('isRootUser', () => {
   expect(isRootUser(null)).toBe(false);
   expect(isRootUser(1001)).toBe(false);
   expect(isRootUser(0)).toBe(true);
 });
+
+test('isFakeRoot', () => {
+  const hasFakerootPreviously = 'FAKEROOTKEY' in process.env;
+  const oldValue = process.env.FAKEROOTKEY;
+  delete process.env.FAKEROOTKEY;
+
+  expect(isFakeRoot()).toBe(false);
+
+  process.env.FAKEROOTKEY = '15574641';
+  expect(isFakeRoot()).toBe(true);
+
+  if (hasFakerootPreviously) {
+    process.env.FAKEROOTKEY = oldValue;
+  }
+});