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/163518Reviewed-by: Jay Conrod <jayconrod@google.com>

test/nosplit.go

@@ -218,6 +218,10 @@ func main() {
 	}
 	defer os.RemoveAll(dir)
 
+	if err := ioutil.WriteFile(filepath.Join(dir, "go.mod"), []byte("module go-test-nosplit\n"), 0666); err != nil {
+		log.Panic(err)
+	}
+
 	tests = strings.Replace(tests, "\t", " ", -1)
 	tests = commentRE.ReplaceAllString(tests, "")