[test] Fix and reenable a Substring.removeSubrange test

Author: lorentey

test/stdlib/StringIndex.swift

@@ -1060,11 +1060,9 @@ suite.test("String.replaceSubrange index validation")
   }
 }
 
-suite.test("Substring.removeSubrange entire range")
-.skip(.always("Disabled due to rdar://99627624"))
-.code {
-  guard #available(SwiftStdlib 5.7, *) else {
-    // This is a regression found in 5.7+
+suite.test("Substring.removeSubrange entire range") {
+  guard #available(SwiftStdlib 5.8, *) else {
+    // This was a regression in 5.7.0, fixed in 5.7.1+
     return
   }