[Test] Disable a flaky test TestThread.test_sleepForTimeInterval

https://bugs.swift.org/browse/SR-15817

Author: rintaro

Tests/Foundation/Tests/TestThread.swift

@@ -173,7 +173,8 @@ class TestThread : XCTestCase {
 			test_callStackReturnAddresses,
                     "Android doesn't support backtraces at the moment."),
 		"And not currently on OpenBSD.")),
-            ("test_sleepForTimeInterval", test_sleepForTimeInterval),
+            ("test_sleepForTimeInterval",
+                testExpectedToFail(test_sleepForTimeInterval, "https://bugs.swift.org/browse/SR-15817")),
             ("test_sleepUntilDate",
                 testExpectedToFail(test_sleepUntilDate, "https://bugs.swift.org/browse/SR-15489")),
             ("test_threadName", test_threadName),