cmd/yacc: move cmd/yacc/expr to cmd/yacc/testdata/expr

This will keep the go command from trying to build it
when the cmd/ tree is no longer a special case.

Also update doc.go to refer to the correct location.
(It was incorrect even before this CL.)

LGTM=r
R=iant, r
CC=golang-codereviews
https://golang.org/cl/134560043
4 files changed