sbt build: split Testing into two parts

the parts didn't really have anything to do with each other, so better
to keep them separate
rename testChecksums task to just ts

to match tr, tc, tm, te
add/update comments + minor code improvement
improve comment
minor cleanup
avoid a small bit of duplication with Scala standard library
DRY up project/FastMediumSlow.scala
Merge remote-tracking branch 'origin/headless' into tortoise-old