Update test_lumi to make the output more useful when things break

Issue #32 (closed) highlighted that the output of test_lumi when it fails isn't currently very useful for debugging.

This MR changes the test such that all events get processed even if an intermediate event causes the test to fail and also adds a print out of a counters values from the first and second decodings when the two values do not agree.

Goes with LHCb!4152 (merged), Allen!1244 (merged)