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
https://golang.org/cl/5622053

test/ken/array.go

@@ -68,6 +68,9 @@ func testpdpd() {
 
 	a = a[5:25]
 	res(sumpd(a), 5, 25)
+
+	a = a[30:95]
+	res(sumpd(a), 35, 100)
 }
 
 // call ptr fixed with ptr fixed