cmd/go: add 'go generate' commands to modfile_flag test

Verify that 'go generate' works with -modfile. Also check that
go commands starts with 'go generate' do not inherit -modfile, but
they should still work if -modfile is set in GOFLAGS.

Updates #34506

Change-Id: I5e1f897b4e38e4fdaccc0fbb7a71b8d0e9fc0660
Reviewed-on: https://go-review.googlesource.com/c/go/+/208236


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>