fixed failing starlette tests

[ricardogsilva]

Overview

This PR modifies the initialization of the starlette test client used in pygeoapi tests in order to comply with
the lastest changes introduced in starlette v0.35.0, specifically the merge of encode/starlette#2400

The change proposed in this PR is to not pass a root_path to the starlette test client, as the new starlette behavior is now to have mounted apps remove the root path when routing - since in the failing tests, which are checking the apirules functionality, pygeoapi is submounting routes under the url_prefix path.

Related issue / discussion

• fixes most recent version of starlette breaks pygeoapi CI #1490

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines