iotests: Introduce $SOCK_DIR

Unix sockets generally have a maximum path length.  Depending on your
$TEST_DIR, it may be exceeded and then all tests that create and use
Unix sockets there may fail.

Circumvent this by adding a new scratch directory specifically for
Unix socket files.  It defaults to a temporary directory (mktemp -d)
that is completely removed after the iotests are done.

(By default, mktemp -d creates a /tmp/tmp.XXXXXXXXXX directory, which
should be short enough for our use cases.)

Use mkdir -p to create the directory (because it seems right), and do
the same for $TEST_DIR (because there is no reason for that to be
created in any different way).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20191017133155.5327-2-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

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