Patch data-types exercises

[philmccarthy]

I recently went through the data-types exercises again as a refresher for mentoring. This PR fixes a few things I found to be confusing or out of place.

I also updated the Advanced Nested Collections exercise to RSpec. I came back to that exercise a few times through Mod 1-2, so I wanted it to remain accessible!

Arrays exercise

• Test 4 - Clarify the setup for adding an element to the end of the array
  • Add prompts for method calls on world_cup array; separate execution from assertion
  • Update assertion to check the last element of the array
• Test 5 - Fix assertion to match exercise directions
  • Instructions say to remove & return (#pop) the last element, but assertion includes "Portugal"
  • Remove "Portugal" from the expected array
• Test 6 - Add prompts for method calls on karaoke to match the "hint" pattern in tests 1-8

Hashes exercise

• Test 4 - Convert dangling minitest assertions to RSpec
  • Fix indentation
  • Match assertion to instructions (asked for a symbol as key but expected a string)

Advanced Nested Collections exercise

• Convert exercise to RSpec
• Move nesting.rb to ./spec/
  • update indentation of stores hash for readability
• Delete nesting_test.rb minitest file
• For reference, I made sure the tests pass as expected. Here are working solutions