Tests: Don't display imported WPT test output if browser is headless

[tcl3]

This PR adds an internals.headless attribute that returns true if the browser is running headlessly and uses it to determine whether imported WPT test output should be shown.

Previously, imported WPT tests didn't display any output if the internals object was exposed. This change adds the condition that the browser must also be running headlessly for test output to not be displayed.

This can be useful when debugging imported WPT tests in cases where the internals object needs to be exposed.