SI-7781 REPL stack trunc shows cause

The handy stack trace truncation in REPL doesn't
show cause like a regular trace.

This commit fixes that and also adds the usual
indicator for truncation, viz, "... 33 more".

The example from the ticket produces:

```
scala> rewrapperer
java.lang.RuntimeException: java.lang.RuntimeException: java.lang.RuntimeException: Point of failure
  at .rewrapper(<console>:9)
  at .rewrapperer(<console>:10)
  ... 32 elided
Caused by: java.lang.RuntimeException: java.lang.RuntimeException: Point of failure
  at .wrapper(<console>:8)
  ... 34 more
Caused by: java.lang.RuntimeException: Point of failure
  at .sample(<console>:7)
  ... 35 more
```

Suppressed exceptions on Java 7 are handled reflectively.

```
java.lang.RuntimeException: My problem
  at scala.tools.nsc.util.StackTraceTest.repressed(StackTraceTest.scala:56)
  ... 27 elided
  Suppressed: java.lang.RuntimeException: Point of failure
    at scala.tools.nsc.util.StackTraceTest.sample(StackTraceTest.scala:29)
    at scala.tools.nsc.util.StackTraceTest.repressed(StackTraceTest.scala:54)
    ... 27 more
```
Target junit.clean to clean junit artifacts

And all.clean will also junit.clean.
SI-7781 Improve test and add comment

The test should normalize the elided message,
not strip it. (Thanks qerub; I was frustrated
with kitteh's turnaround that night, hence
unwilling to improve once it passed.)
SI-7781 Comments to SessionTest

Also, it would be nice if code and expected results were
calculated lazily. That would allow tests with infinite
code but which terminate on various conditions.
Merge pull request #2877 from som-snytt/issue/repl-stack-trunc

SI-7781 REPL stack trunc shows cause