Group common switches in test suite together

Some command line switches are passed to Wget unconditionally. These
switches should exist in a single place instead of being redundantly
defined in each test file. We add the following two switches by default
here:
1. --debug: This causes wget to be most verbose and output a lot of
   debugging information. Hence, if a test fails, the test log should
   provide valuable information.
2. --no-config: Users may have their own wgetrc files on the system.
   However, for the tests, we want Wget to run with vanilla settings.
   Hence, disbale loading any config files.
Minor cleanup in the test suite

The server_conf hook and the server_sett() methods were no longer
required. The server configuration is currently being done by
server_conf() method in the server.