Add test plans support

[maniramezan]

Resolves #442

Short description 📝

This PR is for adding support for parsing and generating test plans feature added in Xcode 11.

Solution 📦

Since it's very similar to existing Testables structure, but much simpler, I followed the same pattern that already existed for Testables. Test plans structure in scheme file is as follow:

...
<TestAction
    ...>
    <TestPlans>
        <TestPlanReference
            reference = "container:uitests.xctestplan"
            default = "YES">
        </TestPlanReference>
        <TestPlanReference
            reference = "container:unittests.xctestplan">
        </TestPlanReference>
    </TestPlans>
    ...
  </TestAction>
  ...

Implementation 👩‍💻👨‍💻

It was pretty straightforward with following the steps for Testables and the adding tests for XML parser. Then added the test plan support to iOS scheme that's used for scheme testing and fixed failing tests.

[pepicrft]

The changes look good @maniramezan. Would you mind updating the CHANGELOG file to mention this improvement? After that we are good to merge 🎉

[maniramezan]

Awesome : ) Added the changes to CHANGELOG file. Let me know if it needed more explanation or links to what test plans.