tests: avoid referring to pvec in demandimport test Nothing else currently uses pvec, so we may want to delete it (but Augie Fackler says he may want to use it). To enable deletion, this patch replaces it by the error module in the demandimport test (any module works). However, since the error module had already been loaded at this point in the test (via the util module), I moved it earlier in the test so it's still not loaded (although I'm not sure if that's even relevant to the test). Differential Revision: https://phab.mercurial-scm.org/D2287