The step summary renders every junit report under build/test-reports,
so the three suites were absent from it for want of a report file. They
now write one under the same name the python runner derives from
INFINITO_TEST_TYPE.
minitest ships no junit reporter, and a gem for one file would be a
dependency the otherwise stdlib-only suite does not need, so it gets a
small plugin instead. The runners also provision their own toolchain
now that the host-side make prerequisites are gone.
