tests/templates: Store config in dataclass

[swick]

The templates are loaded onto the MAIN_OBJ which is the same for all
portal impls. That means if a test case sets up multiple templates, they
all operate on the same object.

If we set, for example, `mock.delay` in both templates, the latter will
overwrite the former.

This commit loads the parameters into a dataclass that's available as
mock.${classname}_params.

Technically we have similar issues whenever we access self in the dbus
method implementations, but for the most part we use different variables
in different templates.

depends on #1607

[whot]

👍