Clean up several minor issues in setPage

[aarongoldenthal]

The following issues were noticed today in the setPage function (all are low priority cleanup).

• Removed unused url argument from setPage
• Moved debug log entry for navigating to URL to proper location (from setPage to gotoUrl)
• Update debug logging for all setPage operations

Debug log changes:

 > pa11y -d https://pa11y.org

 Welcome to Pa11y

  > Running Pa11y on URL https://pa11y.org
  > Debug: Launching Headless Chrome
- > Debug: Opening URL in Headless Chrome
+ > Debug: Opening new browser page
+ > Debug: Setting page User-Agent to "pa11y/8.0.0"
+ > Debug: Setting page viewport
+ > Debug: Navigating to https://pa11y.org
  > Debug: Loading runner: htmlcs
  > Debug: Injecting Pa11y
  > Debug: Injecting runner: htmlcs
  > Debug: Running Pa11y on the page
  > Debug: Document title: "Pa11y"

 No issues found!

[danyalaytekin]

(Oops, let me try posting this again using the review button.)

Well spotted @aarongoldenthal!

1. I think you've completed a reversion which hadn't quite concluded. But this happened a while ago so I should check... are you happy with this signature change @josebolos or was it left there pending a restoration of the reverted feature?
2. Checking that commit I also noticed that we have some testing around the debug logging, which it could make sense to expand to cover the changes in this PR, for consistency, if that's our practice. Alternatively the extra logging could be pushed into a separate PR. @aarongoldenthal @josebolos