go.tools/ssa/interp: remove irrelevent parts of test, improve comments.
go.tools/ssa/interp: remove irrelevent parts of test, improve comments.

(Discovered during diagnosis of bug 7131.)

R=gri
CC=golang-codereviews
https://codereview.appspot.com/52780043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old