autotests: Fix TestXdgShellClientRules

TestXdgShellClientRules implicitly assumes that the kwinrc config is
referenced only by the RuleBook object.

However, after changing the default placement policy in the
WaylandTestApplication, that's no longer the case. The kwinApp() object
now also holds a reference to the main config file. Because of that,
previous window rules leak to next tests, which breaks them.

In order to address that issue, this change makes TestXdgShellClientRules
open a separate config and wipe it clean after each test run. Not great,
but there doesn't seem to be other way around with current KSharedConfig
api.