ToolbarHookToolbarTest has invalid preceding slash in path (and other cleanups)

ToolbarHookToolbarTest does this:

    $this->drupalGet('/node');

The preceding slash is invalid, but since we're not testing the actual content of the page, it's better to decouple it from node (or any specific module or path) and instead use the page testing module intended for this purpose.

Attached fixes that, and has a few textual cleanups as a bonus.