fish: Clarify documentation.

It's not "the guest" which is launched, but the libguestfs appliance.
fish: Split up long line in --help output.
fish: Allow -N filename=type to use 'filename' instead of 'test1.img' (etc.)

So:

  guestfish -N fs

is equivalent to:

  guestfish -N test1.img=fs
fish: For -N option, add drive with explicit format = "raw".

Although autodetecting is safe (we always have just created a drive),
it saves a little bit of time if we don't have to run qemu-img to
detect the disk format with the libvirt backend.

Note that for prepared drives the format is always raw and we don't
anticipate creating drives in other formats.
tests: Use unique or temporary names for temporary files.

Review every test(!) to ensure that it:

 - Doesn't use a generic name (eg. "test1.img", "test.out") for any
   temporary file it needs.

 - Does instead use a unique name or a temporary name (eg. a name like
   "name-of-the-test.img", or a scratch disk).

 - Does not use 'rm -f' to clean up its temporary files (so we can
   detect errors if the wrong temporary file is created or removed).

This allows tests to be run in parallel, so they don't stomp on each
other's temporary files.
Version 1.23.10.