cmd/go: add 'go generate' commands to modfile_flag test
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 <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

cloudflare/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,akutz/go,golang/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,akutz/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,akutz/go