concurrent stacks: add an interface that both stack3 and stack4 implement

Had to slightly tweak (strengthen) stack3's spec for that
2 jobs for master in 9 minutes and 1 second
Status Job ID Name Coverage
  Build
failed #14099
fp
build-coq.8.7.2

00:09:01

failed #14098
fp-timing
build-coq.8.8.0

00:08:43

 
Name Stage Failure
failed
build-coq.8.7.2 Build
Makefile.coq:318: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 3m32.153s
user 12m10.370s
sys 0m13.603s
ERROR: Job failed: exit code 1
failed
build-coq.8.8.0 Build
Makefile.coq:317: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 3m15.110s
user 11m45.107s
sys 0m13.727s
ERROR: Job failed: exit code 1