build: make augeas-gentest.pl write to stdout

build: make augeas-gentest.pl write to stdout

The augeas-gentest.pl program merges a config file into a augeas
file, saving the output to a new file. It is going to be useful
to further process the output file, and it would be easier if this can
be done with a pipeline, so change augeas-gentest.pl to write to stdout
instead of a file.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>