Fixes #201: stop ignoring current subject when inside a .within

[istateside]

What: This PR updates the getContainer util to only fallback to the withinSubject if an explicit subject has not been yielded by the previous command.

Why: The following code fails without this change:

  <section>
    <button>Button Text 1</button>
    <div id="nested">
      <button>Button Text 2</button>
    </div>
  </section>

cy.get('section').within(() => {
  cy.get('#nested').findByText(/Button Text/).should('exist');
});

How: I cloned the repo, added a new test case that failed, and then updated the util to fix the new test.

Checklist:

• [x] Documentation N/A
• [x] Tests
• [x] Ready to be merged

[istateside]

This PR fixes #201

[istateside]

@kentcdodds sorry for the ping, but could I get a review on this?

[github-actions[bot]]

:tada: This PR is included in version 8.0.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: