test: add unit test for part one

internal/2025/DayFour/code_test.go

@@ -0,0 +1,24 @@
+package dayfour
+
+import "testing"
+
+var testInput = []string{
+	"..@@.@@@@.",
+	"@@@.@.@.@@",
+	"@@@@@.@.@@",
+	"@.@@@@..@.",
+	"@@.@@@@.@@",
+	".@@@@@@@.@",
+	".@.@.@.@@@",
+	"@.@@@.@@@@",
+	".@@@@@@@@.",
+	"@.@.@@@.@.",
+}
+
+func TestPartOne(t *testing.T) {
+	expected := 13
+	got := PartOne(testInput)
+	if got != expected {
+		t.Errorf("PartOne() = %d, want %d", got, expected)
+	}
+}