Adds some tests.

Build on 5.3 is failing because of chmod limitation (https://github.com/mikey179/vfsStream/wiki/Filemode)
I guess I did it because phpunit treats warnings, notices, errors as exception and on test https://github.com/joomla-framework/filesystem/pull/7/files#diff-7afb6810062b853bf31dd7fbdaed6166R25 we get a warning "failed to open stream: php_network_getaddresses: getaddrinfo failed".

So I thought if we are returning false (anyway) that would mean something went wrong which would suffice. I didn't know the point you mentioned.

Now looking at I think the better approach would be to either use a test to check the generated warning or suppress it at test (like `$this->assertFalse(@Helper::remotefsize('http://www.joomla.o'))`) and do assertion for false only.