[Tests] Disable a flaky test

rdar://76542113

Author: rintaro

test/Sanitizers/tsan/racy_async_let_fibonacci.swift

@@ -18,6 +18,9 @@
 // SR-15805
 // UNSUPPORTED: CPU=arm64 || CPU=arm64e
 
+// Disabled because this test is flaky rdar://76542113
+// REQUIRES: rdar76542113
+
 func fib(_ n: Int) -> Int {
   var first = 0
   var second = 1