Add test coverage for pkg/tarsum

And removing unused code.
- tarsum.go :
  NewTarSumHash could be non exported (for now)
  NewTarSumForLabel is never used, except for the tests
- fileinfosums.go:
  SortByPos is never used, except for the tests
- versionning.go:
  GetVersions is never used, expect for the tests

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

tarsum/fileinfosums_test.go

@@ -42,4 +42,21 @@ func TestSortFileInfoSums(t *testing.T) {
 	if gotFis.Pos() != 4 {
 		t.Errorf("Expected %d, got %d", 4, gotFis.Pos())
 	}
+
+	fis = newFileInfoSums()
+	fis.SortByPos()
+	if fis[0].Pos() != 0 {
+		t.Errorf("sorted fileInfoSums by Pos should order them by position.")
+	}
+
+	fis = newFileInfoSums()
+	expected = "deadbeef1"
+	gotFileInfoSum := fis.GetFile("dup1")
+	if gotFileInfoSum.Sum() != expected {
+		t.Errorf("Expected %q, got %q", expected, gotFileInfoSum)
+	}
+	if fis.GetFile("noPresent") != nil {
+		t.Errorf("Should have return nil if name not found.")
+	}
+
 }