[SV 63856] Fix pruning of double-colon rules

[SV 63856] Fix pruning of double-colon rules

Given this setup:

  $ cat Makefile
  A::; @echo A-1 && sleep 1 && echo A-1 done
  A::; @echo A-2 && sleep 1 && echo A-2 done
  A::; @echo A-3 && sleep 1 && echo A-3 done
  B::; @echo B-1 && sleep 1 && echo B-1 done
  B::; @echo B-2 && sleep 1 && echo B-2 done
  B::; @echo B-3 && sleep 1 && echo B-3 done

  $ make -j8 A .WAIT B

All recipes for A should be started sequentially and complete before
any recipe for B is started, then all recipes for B should be started
sequentially.  This wasn't happening because the double-colon target
was getting pruned too early.

* src/remake.c (update_file): Don't prune a target if it's a double
colon rule which is complete, but there are other recipes to run for
this target: we want those other recipes to be run first.
* tests/scripts/targets/WAIT: Test .WAIT with double colon rules.