libusbgx: tests: Add simulation of configfs for gadget attrs get/set

libusbgx: tests: Add simulation of configfs for gadget attrs get/set

Before executing libusbgx functions which get and set gadget attributes,
we must prepare virtual filesystem to behave as expected.
push_* and pull_* functions take arguments similar to usbg functions
which will be called after them, e.g. push_gadget_attrs(g, a)
will prepare for calling usbg_get_gadget_attrs on gadget matching g,
where a is value passed to be read.

Change-Id: I805544dbd35a1a7f8c2e4073f9f4607d9c70bb95
Signed-off-by: Pawel Szewczyk <p.szewczyk@samsung.com>
[Update description]
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>