tests: don't segfault if gpio-sim is unavailable

tests: don't segfault if gpio-sim is unavailable

The GLib wrapper for libgpiosim tries to create the gpiosim context and
bank objects in the class's init() function but the constructed()
callback doesn't check if that operation succeeded, leading to a crash
when it dereferences the bank pointer.

As init() should only perform operations that cannot fail, let's move all
the gpiosim initialization code to constructed() and bail-out of it if
any of the steps fails.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>