RNTesterIntegrationTests: Fix testTimersTest failure

[hramos]

Help Wanted: Fix TimersTests

test_objc on Circle CI is red due to a failing integration test, TimersTest:

UI Test Activity: 
Assertion Failure: RCTTestRunner.m:233: failed: caught "NSInternalInconsistencyException", "RedBox errors: (
    "undefined is not an object (evaluating 'this.setTimeout')",
    "Unhandled JS Exception: undefined is not an object (evaluating 'this.setTimeout')"
)"

The test is defined in JavaScript at IntegrationTests/TimersTest.js, and is invoked by RNTesterIntegrationTests.

Steps to reproduce:

1. Run yarn at the repository root.
2. Open RNTester/RNTester.xcodeproj on Xcode 10.1.0.
3. Build RNTester.
4. Run tests (CMD+U) on an iPhone 6 simulator.