internal/report: remove use of CutPrefix and add test for labels

Using new(er) function CutPrefix caused deploy to fail because it expected
to run using older version of Go.

Change-Id: I335ca27870caf0b9d6a57bf98fb3e2be1e205743
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/540855
Auto-Submit: Tatiana Bradley <tatianabradley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

internal/report/report.go

@@ -104,7 +104,10 @@ func (er ExcludedReason) ToLabel() string {
 }
 
 func FromLabel(label string) (ExcludedReason, bool) {
-	er, ok := strings.CutPrefix(label, excludedLabelPrefix)
+	pre, er, ok := strings.Cut(label, excludedLabelPrefix)
+	if pre != "" {
+		return "", false
+	}
 	return ExcludedReason(er), ok
 }
 

internal/report/report_test.go

@@ -75,6 +75,19 @@ func TestYAMLFilename(t *testing.T) {
 	}
 }
 
+func TestToFromLabel(t *testing.T) {
+	str := "EFFECTIVELY_PRIVATE"
+	label := "excluded: EFFECTIVELY_PRIVATE"
+	er := ExcludedReason(str)
+	if got, want := er.ToLabel(), label; got != want {
+		t.Errorf("(%s).ToLabel = %s, want %s", er, got, want)
+	}
+	got, ok := FromLabel(label)
+	if want := er; !ok || got != want {
+		t.Errorf("FromLabel(%s) = (%s, %t), want (%s, true)", label, got, ok, want)
+	}
+}
+
 func TestParseFilepath(t *testing.T) {
 	filepath := "data/reports/GO-1999-0023.yaml"
 	wantFolder := "data/reports"