go/zodb/fs1tools: tests: Use fresh dumper for each subtest

We were reusing Dumper instance in between testDump subtests. This was
not noticed, as it was only "1" and then "empty" case, because "emtpy"
has no transactions. However in the next patch we'll add another
subcase, and if the dumper instance is not reset, it will think that it
starts from transaction number non-zero, which would differ from fresh
dumper output.

-> Fix it.

go/zodb/storage/fs1/fs1tools/dump_test.go

-// Copyright (C) 2017  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
@@ -48,10 +48,11 @@ func loadFile(t *testing.T, path string) string {
 	return string(data)
 }
 
-func testDump(t *testing.T, dir fs1.IterDir, d Dumper) {
+func testDump(t *testing.T, dir fs1.IterDir, newd func() Dumper) {
 	testv := []string{"1", "empty"}
 	for _, tt := range testv {
 		t.Run("db=" + tt, func(t *testing.T) {
+			d   := newd()
 			buf := bytes.Buffer{}
 
 			err := Dump(&buf, fmt.Sprintf("../testdata/%s.fs", tt), dir, d)
@@ -68,9 +69,13 @@ func testDump(t *testing.T, dir fs1.IterDir, d Dumper) {
 	}
 }
 
-func TestFsDump(t *testing.T)	{ testDump(t, fs1.IterForward,  &DumperFsDump{}) }
-func TestFsDumpv(t *testing.T)	{ testDump(t, fs1.IterForward,  &DumperFsDumpVerbose{}) }
-func TestFsTail(t *testing.T)	{ testDump(t, fs1.IterBackward, &DumperFsTail{Ntxn: 1000000}) }
+func newFsDump()  Dumper { return &DumperFsDump{} }
+func newFsDumpv() Dumper { return &DumperFsDumpVerbose{} }
+func newFsTail()  Dumper { return &DumperFsTail{Ntxn: 1000000} }
+
+func TestFsDump(t *testing.T)	{ testDump(t, fs1.IterForward,  newFsDump) }
+func TestFsDumpv(t *testing.T)	{ testDump(t, fs1.IterForward,  newFsDumpv) }
+func TestFsTail(t *testing.T)	{ testDump(t, fs1.IterBackward, newFsTail) }
 
 func BenchmarkTail(b *testing.B) {
 	// FIXME small testdata/1.fs is not representative for benchmarking