This option, when added via
  TESTS_ENVIRONMENT = [...] $(top_builddir)/run --test
allows us to run the tests and only print the full output (including
debugging etc) when the test fails.

The test uses FUSE, so we need a way to disable it on RHEL 5.

Old KVM can't add /dev/null readonly.  Treat /dev/null as a special
case.

We also fix a few tests where /dev/null was being used with
format=qcow2.  This was always incorrect behaviour, but qemu appears
to tolerate it.

RHEL 5 didn't have the truncate command, but we can replace:

  truncate -s SIZE FILE

with the roughly equivalent command:

  guestfish sparse FILE SIZE

This API makes device names canonical, eg. /dev/vda1 -> /dev/sda1.

The test used the external 'od' command to compare the output of
guestfish with what it's supposed to be.  Unfortunately by default
this outputs groups of 2-byte words, with the words' endianness
affected by the current hardware endianness.  For example:

x86-64$ echo -n ab | od
0000000 061141
0000002

ppc64$ echo -n ab | od
0000000 060542
0000002

By using 'od -b' instead we can output bytes instead of words, and
there is no endianness issue, and the output is clearer:

x86-64$ echo -n ab | od -b
0000000 141 142
0000002

ppc64$ echo -n ab | od -b
0000000 141 142
0000002

For example:

><fs> glob echo /dev/*
/dev/vda
/dev/vda1
/dev/vda2
/dev/vda3
><fs> glob echo /dev/v*/*
/dev/vg_f16x64/lv_root
/dev/vg_f16x64/lv_swap