go/internal/xtesting: Fix FatalIf to return a helper

Else, on an error, it is the lineno of `t.Fatal(err)` inside FatalIf
that is printed, not the line number inside user test.

go/internal/xtesting/xtesting.go

-// Copyright (C) 2017-2020  Nexedi SA and Contributors.
+// Copyright (C) 2017-2021  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -445,6 +445,7 @@ func DrvTestWatch(t *testing.T, zurl string, zdrvOpen zodb.DriverOpener) {
 // FatalIf(t) returns function f(err), which call t.Fatal if err != nil.
 func FatalIf(t *testing.T) func(error) {
 	return func(err error) {
+		t.Helper()
 		if err != nil {
 			t.Fatal(err)
 		}