partest: put more stuff on javac classpath
partest: put more stuff on javac classpath

Some java tests require the scala-library to be present on the
classpath, this fixes tests/pos/java-interop/{t1186, t1235, t1254,
t1642}.
Also correctly redirect the output of javac so that it will be displayed
by partest --verbose

som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,som-snytt/dotty,lampepfl/dotty,som-snytt/dotty,sjrd/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,som-snytt/dotty,dotty-staging/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty