[test] Disable test on Linux that's ICU version-specific

Author: milseman

test/stdlib/CharacterProperties.swift

@@ -142,7 +142,14 @@ CharacterPropertiesTests.test("Casing") {
   expectTrue(Character("π").isLowercase)
 
   expectEqual("SS", Character("ß").uppercased())
+
+// Some versions of ICU on Linux (62.1) have a bug producing the wrong value
+// when lowercasing "ẞ". Darwin platforms never shipped this version, so
+// conditionally test based on platform.
+#if _runtime(_ObjC)
   expectEqual("ß", Character("ẞ").lowercased())
+#endif
+
   expectEqual("и", Character("И").lowercased())
   expectEqual("И", Character("и").uppercased())
   expectEqual("π", Character("Π").lowercased())