testscript: tip broke MainStart for fuzzing support #144
Description
# github.com/rogpeppe/go-internal/testscript
go/pkg/mod/github.com/rogpeppe/go-internal@v1.8.1-0.20210531173208-9df06c38c09b/testscript/exe.go:231:8: not enough arguments in call to testing.MainStart
Looks like the merge of dev.fuzz last night added a parameter to MainStart. We should probably use build tags to insert an extra nil parameter on Go 1.18 and later.