test: add unit test for p1 from paper calculation

internal/2015/DaySix/code_test.go

@@ -0,0 +1,17 @@
+package daysix
+
+import "testing"
+
+var testInput = []string{
+	"turn on 0,0 through 999,999",
+	"toggle 0,0 through 999,0",
+	"turn off 499,499 through 500,500",
+}
+
+func TestPartOne(t *testing.T) {
+	expected := 998996
+	got := PartOne(testInput)
+	if got != expected {
+		t.Errorf("PartOne() = %d, want %d", got, expected)
+	}
+}