Investigate `sinter collect` only collecting for 1 decoder when multiple specified

[Strilanc]

While collecting for heavy hex demo I noticed that an invocation of sinter collect with two decoders only ran cases for one of the decoders. Not sure what exactly triggered it. Some work was done and some was not when invoking. It looked like it correctly counted how many variants to run, but all variants for the other decoder were skipped as if finished.