modfile: add retract directives to TestParseLax

Old versions of modfile before CL 228039 should be able to parse and
ignore retract directives with version intervals.

Updates golang/go#24031

Change-Id: I0f35261997c0704a785ea8467dc5ed0738549966
Reviewed-on: https://go-review.googlesource.com/c/mod/+/230697
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
1 file changed