tests/qapi-schema: Tidy up pylint warnings and advice

Pylint warns:

    tests/qapi-schema/test-qapi.py:139:13: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)
    tests/qapi-schema/test-qapi.py:143:13: W1514: Using open without explicitly specifying an encoding (unspecified-encoding)

Add encoding='utf-8'.

Pylint advises:

    tests/qapi-schema/test-qapi.py:143:13: R1732: Consider using 'with' for resource-allocating operations (consider-using-with)

Silence this by returning the value directly.

Pylint advises:

    tests/qapi-schema/test-qapi.py:221:4: R1722: Consider using sys.exit() (consider-using-sys-exit)
    tests/qapi-schema/test-qapi.py:226:4: R1722: Consider using sys.exit() (consider-using-sys-exit)

Sure, why not.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20231025092925.1785934-1-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>