Make unit tests greenbar on Linux

This PR has several changes that makes the unit tests on Linux greenbar. It includes some necessary enhancements (Jenkins build agent requires proxy) and ignores some other tests (listed in BL-831).
These are very good tests, but one suggestion: the style we mostly use in Bloom is to give tests longer names indicating what happens, e.g., ProxyNoPort_DefaultsToPort80 or BothEnvironmentVariables_UsesLCVersion. Not essential, but might be worth considering.