perlPackages.LWPUserAgentMockable: Disable tests

These tests need network connectivity, which isn't available in a
chroot'ed environment (at least, it's not available on Hydra build machines).
Merge pull request #1591 from ocharles/perlPackages.LWPUserAgentMockable

perlPackages.LWPUserAgentMockable: Disable tests