Add helper function for Duration parsing

Tests in cmd_forget_test.go need the same convenience function
that was implemented in snapshot_policy_test.go.

Function parseDuration(...) was moved to testing.go  and renamed to
ParseDurationOrPanic(...).

internal/restic/testing.go

@@ -212,3 +212,14 @@ func TestParseHandle(s string, t BlobType) BlobHandle {
 func TestSetSnapshotID(t testing.TB, sn *Snapshot, id ID) {
 	sn.id = &id
 }
+
+// Convenience function that parses a duration from a string or panics if string is invalid.
+// The format is `6y5m234d37h`.
+func ParseDurationOrPanic(s string) Duration {
+	d, err := ParseDuration(s)
+	if err != nil {
+		panic(err)
+	}
+
+	return d
+}