test: add unit test for part one

internal/2015/DayFifteen/code_test.go

@@ -0,0 +1,16 @@
+package dayfifteen
+
+import "testing"
+
+var testInput = []string{
+	"Butterscotch: capacity -1, durability -2, flavor 6, texture 3, calories 8",
+	"Cinnamon: capacity 2, durability 3, flavor -2, texture -1, calories 3",
+}
+
+func TestPartOne(t *testing.T) {
+	expected := 62842880
+	got := PartOne(testInput)
+	if got != expected {
+		t.Errorf("PartOne() = %d, want %d", got, expected)
+	}
+}