build-aux: rewrite augeas test generator in Python

build-aux: rewrite augeas test generator in Python

As part of an goal to eliminate Perl from libvirt build tools,
rewrite the augeas-gentest.pl tool in Python.

This was a straight conversion, manually going line-by-line to
change the syntax from Perl to Python. Thus the overall structure
of the file and approach is the same.

The use of $(AUG_GENTEST) as a dependancy in the makefiles needed
to be fixed, because this was assumed to be the filename of the
script, but is in fact a full shell command line.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>