Use Char.MIN_VALUE and MAX_VALUE in tests

compiler/testData/codegen/box/binaryOp/overflowChar.kt

@@ -1,10 +1,11 @@
 fun box(): String {
-    val c1: Char = 0.toChar()
+    val c1: Char = Char.MIN_VALUE
     val c2 = c1 - 1
-    if (c2 < c1) return "fail: 0.toChar() - 1 should overflow to positive."
+    if (c2 < c1) return "fail: Char.MIN_VALUE - 1 should overflow to positive."
 
-    val c3 = c2 + 1
-    if (c3 > c2) return "fail: FFFF.toChar() + 1 should overflow to zero."
+    val c3: Char = Char.MAX_VALUE
+    val c4 = c3 + 1
+    if (c4 > c3) return "fail: Char.MAX_VALUE + 1 should overflow to zero."
 
     return "OK"
 }