Warn if no write permissions when executing Chebgui export tests.

David Watkins sent us an e-mail indicating that these tests were failing on his machine, and while we don't have full confirmation, it looks like this is due to the fact that he has Chebfun installed in a directory to which his user account cannot write.  These tests now check if writing is allowed and pass automatically with a warning if it isn't.

Thanks to @nickhale for suggesting an idea similar to this.