builtin: add tests for rune.to_upper() (#22801)

vlib/builtin/rune_test.v

@@ -40,3 +40,36 @@ fn test_bytes() {
 	r1 := `★`
 	assert r1.bytes() == [u8(0xe2), 0x98, 0x85]
 }
+
+fn test_to_upper() {
+	assert `c`.to_upper() == `C`
+	assert `C`.to_upper() == `C`
+	assert `δ`.to_upper() == `Δ`
+	assert `Δ`.to_upper() == `Δ`
+	assert `ā`.to_upper() == `Ā`
+	assert `Ā`.to_upper() == `Ā`
+	assert `Я`.to_upper() == `Я`
+	assert `я`.to_upper() == `Я`
+}
+
+fn test_to_lower() {
+	assert `C`.to_lower() == `c`
+	assert `c`.to_lower() == `c`
+	assert `Δ`.to_lower() == `δ`
+	assert `δ`.to_lower() == `δ`
+	assert `Ā`.to_lower() == `ā`
+	assert `ā`.to_lower() == `ā`
+	assert `я`.to_lower() == `я`
+	assert `Я`.to_lower() == `я`
+}
+
+fn test_to_title() {
+	assert `c`.to_title() == `C`
+	assert `C`.to_title() == `C`
+	assert `δ`.to_title() == `Δ`
+	assert `Δ`.to_title() == `Δ`
+	assert `ā`.to_title() == `Ā`
+	assert `Ā`.to_title() == `Ā`
+	assert `я`.to_title() == `Я`
+	assert `Я`.to_title() == `Я`
+}