Fix path construction for report file.

The second argument passed to startNetTest() was not a string as
expected.  Also, the standard reports directory should be used to store
reports instead of the current directory ".".

This should fix the reporting component of ooniprobe's HTTP API.
Use the get() method to obtain the value.

This is a problem when the code is reached from entry points other than
ooniprobe (such as oonid) as this is the only place where argument
parsing for no-geoip is done.
Return interface of default route.

As the name of the method suggests, the default interface should be
returned and not the interface of the first found private network.
Also, add a meaningful error message to the raised exception.
Fix XSRF protection.

This fix correctly extracts the XSRF token from the HTTP header and
raises exceptions if anything fails in the process.
Fix detection for already existing files.

The fix first builds absolute paths and then runs the check on it.
Besides, the fix adds meaningful error messages for raised exceptions.
Use correct file paths in unit tests.
Other fixes to filepaths to make nettest unittests pass.