Add Regression test for Scala Bug 7131

[diesalbla]

Fixes scala/bug#7131, in so far as the compiler no longer crashes.

The program in t7131.scala caused the compiler to crash with StackOverflow This has been solved before version 2.13.x and 2.12.7, and the compiler no longer crashes. In This PR adds the program in question, to watch for regressions. We add the compile error generated in the current version.

Although the compiler no longer crashes, the output is different between versions 2.13.x and 2.12.7. In the latter, it reports one error, on the definition of the ObservableValue trait:

t7131.scala:5: error: contravariant type T occurs in covariant position in type => T of method value
  def value: T

Instead, current version 2.13.x misses that error, and instead reports the error noted in the t7131.check file. This may indicate another compiler bug, since the variance error above should also be reported. Interestingly, when writing that trait into the REPL, the 2.13.x does reject it.

Welcome to Scala 2.13.0-20181116-133635-1f76b24 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_172).

scala> trait Ann[- X]{ def value: X}
                           ^
       error: contravariant type X occurs in covariant position in type => X of method value

[diesalbla]

/rebuild

[som-snytt]

Needs a check file update:

[test] -t7131.scala:27: error: type mismatch;
[test] +t7131.scala:21: error: type mismatch;

[som-snytt]

It would be great if jenkins could do these updates automatically.

You'll also need to squash the commits, since every commit must pass.