Idempotency tests should use forked JVMs

[smarter]

Currently, https://github.com/lampepfl/dotty/blob/master/compiler/test/dotty/tools/dotc/IdempotencyTests.scala calls compileFilesInDir and compieList which will run Dotty in the current JVM, but some instabilities may only become apparent when the same code is compiled using different JVMs (in paticular because it means that all objects will get different identityHashCode, which will lead to sets being ordered and traversed in different orders, etc). So to make the idempotency tests more robust they should start a new java process each time they want to compile the same code again (different pieces of code can still be compiled with the same JVM to keep the tests fast).

[ashwinbhaskar]

@smarter I am trying to run just the tests in IdempotencyTests.scala using sbt "testOnly compiler/test/dotty/tools/dotc/IdempotencyTests.scala". But no tests are run.

[smarter]

testOnly takes a class name, not a file name, so you need to run testOnly *IdempotencyTests from sbt.