efi_loader: fix status management in network stack

The network should start in status EfiSimpleNetworkStopped.

Add and correct status checks in the simple network protocol.

Correct the unit test:
* Shutdown() and Stop() during setup if needed
* invoke Shutdown() before Stop() when tearing down

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>