test: add unit test for part one

internal/2015/DayNine/code_test.go

@@ -0,0 +1,17 @@
+package daynine
+
+import "testing"
+
+var testInput = []string{
+	"London to Dublin = 464",
+	"London to Belfast = 518",
+	"Dublin to Belfast = 141",
+}
+
+func TestPartOne(t *testing.T) {
+	expected := 605
+	got := PartOne(testInput)
+	if got != expected {
+		t.Errorf("PartOne() = %d, want %d", got, expected)
+	}
+}