Can we change the tramp test suite so that, by default, it doesn't try
to access the network? If a host isn't connected to the network, tests
can hang for a long time with no user-visible explanation, and even if
network connectivity is available, ssh to the local system isn't
guaranteed to work, or work without asking for input. (The tramp tests
constantly ask me whether I want to connect.)

It'd also be nice to give Emacs /dev/null as its stdin when it's running
the test suite. Is there any reason we want tests to be able to interact
with the user?