tests: disable shutdown and hysteresis for Cygwin

It seems that the hysteresis test fails every now and then, presumably
due to timing issues. The tests needs some improvements, e.g. isolating
it from the system time.

platforms/posix/cmake/sitl_tests.cmake

@@ -44,7 +44,9 @@ endif()
 
 if (CMAKE_SYSTEM_NAME STREQUAL "CYGWIN")
 	list(REMOVE_ITEM tests
+		hysteresis # Intermittent timing fails.
 		uorb
+		shutdown # A mystery why it fails.
 	)
 endif()