test: add a go.mod file in the working directory of nosplit.go
test: add a go.mod file in the working directory of nosplit.go

Updates #30228

Change-Id: I41bbedf15fa51242f69a3b1ecafd0d3191271799
Reviewed-on: https://go-review.googlesource.com/c/163518
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

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