Test amcssth fails very occasionally

[rptb1]

See https://app.travis-ci.com/github/Ravenbrook/mps/jobs/593083274#L899-L908

We know from long experience that this is very unlikely to be a bug in the MPS, but something wrong with the test case.

However, a failure like this obstructs progress, as it did today while trying to merge,

[rptb1]

Duplicate of #59