Extension/contraction callback test does not check correlations

[rptb1]

In https://github.com/Ravenbrook/mps/pull/213#pullrequestreview-1422468433 @UNAA008 says:

q .limitation on not checking numbers of extensions and contractions. Is this a dangerous omission?

Not very dangerous. The test could be altered to use manual allocation and try to correlate allocations and frees with extensions and contractions, as a form of white box testing.