cmd/go: remove tests that assume lack of new versions of external modules
In general it seems ok to assume that an open-source module that did
exist will continue to do so — after all, users of open-source modules
already do that all the time. However, we should not assume that those
modules do not publish new versions — that's really up to their
maintainers to decide.
Two existing tests did make that assumption for the module
gopkg.in/natefinch/lumberjack.v2. Let's remove those two tests.
If we need to replace them at some point, we can replace them with
hermetic test-only modules (#54503) or perhaps modules owned by the Go
project.