Fix Flaky Tests / Optimize Checking of `plan.title` Within `spec/features/plans/exports_spec.rb`

[aaronskiba]

Fixes #3450

Changes proposed in this PR:

• Fixes the flaky tests within spec/features/plans/exports_spec.rb (see #3450)
  • While inspecting these sometimes failing tests, it was found that expect(page.source).to have_text(plan.title) sometimes failed because sometimes page.source == "".
  • Rather than page.source, which returns the entire HTML content of the page, this PR uses page.title, which only returns the contents inside of the <title> tags.
    • page.title does not seem to encounter the unwanted behaviour of returning a blank string. Maybe because it is faster (only returning the title should be faster than returning the entire HTML content via page.source)?
    • Also, the <title> title tags and their contents are part of the entire HTML content. So despite page.source returning a blank string, because page.title is not blank, it follows that the DOM is not blank.