Diskless clients can now run the full atf test suite

[Martin Husemann <martin%duskware.de@localhost>]

It maybe a suprise for you, but atf runs of our test suite didn't work
before with / on NFS. Some tests failed and could fill up /tmp (if on
tmpfs), which then would cause more test failures. See PR bin/48892 for
an example and more details.

Of course this was due to bugs - and those have now been fixed, at least
my recent evbarm test run (on a diskless GuruPlug) had no spurious fallout.

This is nice, as setting up a test environment on an NFS server is easy
and now we can quickly do full test runs without touching the system
installed on the test machine itself.

Martin