[test] Don't check for "HK$" in stdlib/TestLocale.swift on older OS X

Apparently this changed at some point in the past (it used to just be
"$"). The actual value isn't so important, so just skip that part.

Part of rdar://problem/44866579

Author: jrose-apple

test/stdlib/TestLocale.swift

@@ -94,7 +94,9 @@ class TestLocale : TestLocaleSuper {
 
         expectEqual(".", locale.decimalSeparator)
         expectEqual(",", locale.groupingSeparator)
-        expectEqual("HK$", locale.currencySymbol)
+        if #available(macOS 10.11, *) {
+          expectEqual("HK$", locale.currencySymbol)
+        }
         expectEqual("HKD", locale.currencyCode)
 
         expectTrue(Locale.availableIdentifiers.count > 0)