tests: don't use g_value_set_static_string() for non-static strings

tests: don't use g_value_set_static_string() for non-static strings

As pointed out by Philip Withnall, g_value_set_static_string() must only
be used with actual static strings and not with ones whose life-time is
tied to that of their owner. Use g_value_set_string() to get the gpiosim
properties and rework the existing getter functions returning const
gchar * to return the address provided by libgpiosim directly instead of
passing through the GObject property path.

Suggested-by: Philip Withnall <philip@tecnocode.co.uk>
Link: https://lore.kernel.org/r/20240813093025.94980-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>