Update existing versionbits_tests.cpp
Update existing versionbits_tests.cpp

The only changes here are needed to
a) make it work given that we will update the AbstractThresholdConditionChecker (additional member functions provided in a subsequent commit)
b) a fix to the disjointness check, which previously worked on the unwritten assumption that the deployments would be ordered in a way that would reduce effort needed to check disjointness. With the opening up of the configuration interface, and even before then, this is a bad assumption, so the check was generalized to compare each fork against all the others.

No changes to the rest of the test's logic have been made, to ensure it still tests correctly, to fulfil the backward compatibility goal of bip-genvbvoting.

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