test/io: skip tests not supported when using synchronous IO

tests/io/CancelFutureTest.cpp

tests/io/ConcurrentNetworkEchoTest.cpp

tests/io/FutureCallbackTest.cpp

tests/io/IncrementalCompletionTest.cpp

tests/io/LinkFutureTest.cpp

tests/io/SimpleDiskAndNetworkTest.cpp

tests/io/TimeoutTest.cpp

tests/io/TimeoutWrapperTest.cpp

tests/io/TooLongFutureChain.cpp