test: test slice beyond len
test: test slice beyond len

When slicing a slice, the bounds may be > len as long as they
are <= cap.  Interestingly, gccgo got that wrong and still
passed the testsuite and all the library tests.

R=golang-dev, rsc, iant
CC=golang-dev
http://codereview.appspot.com/5622053

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang