tests: Use medium dataset for perlbmk regressions

tests: Use medium dataset for perlbmk regressions

This patch changes the perlbmk regression script from the large to the
medium dataset to reduce the regression run time. For all ISAs and CPU
models, the total perlbmk host CPU time with the large dataset is
roughly 12 hours (constituting >30% of the total regression host
time). There is, most likely, almost no added value in terms of code
coverage for this rather excessive run time.