Add updateCheckFiles to override tests check files with output

[nicolasstucki]

• Also updated testFromTasty checkfiles using it to remove first comment line
  which cannot be tested on Windows adn added missing EOLs.
• Add --update-checkfiles to testCompilation
• Replace testFromTasty with testCompilation --from-tasty

[smarter]

• We need a way to run this from the sbt command-line, I suggest testCompilation --update-check (by the way, I also think testFromTasty should be replaced with a flag passed to testCompilation).
• In Scala 2's partest, the summary at the end gives you the exact command to run to update all failed check files (e.g. partest --update-check tests/run/foo.scala tests/run/bla.scala)
• This needs to be documented in https://github.com/lampepfl/dotty/blob/master/docs/docs/contributing/workflow.md

[smarter]

Awesome! Last thing that could be useful would be a --help for testCompilation if you have time :)

[nicolasstucki]

Ping @biboudis note the change with testFromTasty