qemu-iotests: iotests: fail hard if not run via "check"

Running an iotests-based Python test directly might appear to work,
but may fail in subtle ways and is insecure:

- It creates files with predictable file names in a world-writable
  location (/var/tmp).

- Tests expect the environment to be set up by check. E.g. 041 and 055
  may take the wrong code paths if QEMU_DEFAULT_MACHINE is not
  set. This can lead to false negatives.

Instead fail hard and tell the user we want to be run via "check".

The actual environment expected by the tests is currently only defined
by the implementation of "check". We use two of the environment
variables set by "check" as indication of whether we're being run via
"check". Anyone writing their own test runner (replacing "check") will
need to replicate the full environment (in a broader sense, not just
environment variables) provided by "check" anyway, including setting
the two environment variables we check. Whereas a regular developer
just trying to invoke the tests usually won't have both of these
defined in their environment so we can catch their mistake and give
out useful advice.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Bo Tu <tubo@linux.vnet.ibm.com>
Message-id: 1461094442-16014-1-git-send-email-silbe@linux.vnet.ibm.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
