[Test][Backtracing] Only disable FatalError for macOS.

Actually, we only need to disable this for macOS.  It should be fine
on Linux.

Author: al45tair

test/Backtracing/FatalError.swift

@@ -8,8 +8,7 @@
 // UNSUPPORTED: asan
 // REQUIRES: executable_test
 // REQUIRES: backtracing
-// REQUIRES: OS=macosx || OS=linux-gnu
-// REQUIRES: rdar116790881
+// REQUIRES: OS=linux-gnu
 
 func level1() {
   level2()