tests: test behavior of IOError during transactions (issue5658) ui._write(), ui._write_err(), and ui.flush() all trap IOError and re-raise as error.StdioError. If a caller doesn't catch StdioError when writing to stdio, it could bubble all the way to dispatch. This commit adds tests for I/O failures around various transaction operations. The most notable badness is during abort. Here, an uncaught StdioError will result in incomplete transaction rollback, requiring an `hg rollback` to recover. This can result in a client "corrupting" a remote repo via terminated HTTP and SSH socket.