tests: fix RRDs deps in unit testing

RRDs.pm does exist in travis. We do not really use it anyway.
tests: fix warning about potential typo
tests: Net::Server dep was missing in .travis.yml
tests: Module::Build is duplicated in .travis.yml
tests: rm the OS::get_fq_hostname() test for "."

The Travis-CI VM do not have any "." in their FQDN.