exp/ssa/interp: (#6 of 5): test interpretation of SSA form of $GOROOT/test/*.go.

The interpreter's os.Exit now triggers a special panic rather
than kill the test process.  (It's semantically dubious, since
it will run deferred routines.)  Interpret now returns its
exit code rather than calling os.Exit.

Also:
- disabled parts of a few $GOROOT/tests via os.Getenv("GOSSAINTERP").
- remove unnecessary 'slots' param to external functions; they
  are never closures.

Most of the tests are disabled until go/types supports shifts.
They can be reenabled if you patch this workaround:
https://codereview.appspot.com/7312068

R=iant, bradfitz
CC=golang-dev, gri
https://codereview.appspot.com/7313062