Fragile tests should compile

The test concprog001 is marked as fragile and fails on CI with

Converter.hs:113:35: error:
    Ambiguous occurrence ‘yield’
    It could refer to
       either ‘GHC.Data.Stream.yield’,
              imported from ‘GHC.Data.Stream’ at Converter.hs:4:1-22
           or ‘Control.Concurrent.yield’,
              imported from ‘Control.Concurrent’ at Converter.hs:6:1-25
              (and originally defined in ‘GHC.Conc.Sync’)

This is a concurrency test. I doubt that it is flaky because of an ambiguous import.

We should require that fragile tests compile successfully, otherwise they should fail loudly.