test: add fallbackModes unit tests

internal/playback/watch_data_test.go

@@ -0,0 +1,35 @@
+package playback
+
+import (
+	"testing"
+)
+
+func TestFallbackModes(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		mode string
+		want []string
+	}{
+		{name: "sub falls back to dub", mode: "sub", want: []string{"dub"}},
+		{name: "dub falls back to sub", mode: "dub", want: []string{"sub"}},
+		{name: "unknown tries both canonical modes", mode: "raw", want: []string{"sub", "dub"}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := fallbackModes(tt.mode)
+			if len(got) != len(tt.want) {
+				t.Fatalf("len(got) = %d, want %d", len(got), len(tt.want))
+			}
+			for i, want := range tt.want {
+				if got[i] != want {
+					t.Fatalf("got[%d] = %q, want %q", i, got[i], want)
+				}
+			}
+		})
+	}
+}