scons: fix minor update-ref bug in regressions

scons: fix minor update-ref bug in regressions

In the unusual case that regressions are run with --update-ref
when there is no existing regression output, scons gets
confused because it depends on stats.txt to trigger the
update, but it has no indication that running the test will
generate the stats.txt file.  (In the typical case where
stats.txt already exists, scons doesn't care about where
it came from.)

It's easy to fix this just by adding the stats.txt file
to the target list for the test action.