"faster version of ClosedSubsets" broke test suite

[fingolfin]

With commit d9ad0a5, the SLA test suite got broken. On https://travis-ci.org/gap-packages/sla/jobs/502455062 we see:

...
testing: /home/travis/build/gap-packages/sla/gaproot/pkg/sla/tst/sla05.tst
########> Diff in /home/travis/build/gap-packages/sla/gaproot/pkg/sla/tst/sla0\
5.tst:126
# Input is:
c:= ClosedSubsets(R);;
# Expected output:
# But found:
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `Dimension' on 1 arguments
########
########> Diff in /home/travis/build/gap-packages/sla/gaproot/pkg/sla/tst/sla0\
5.tst:127
# Input is:
Length(c);
# Expected output:
4844
# But found:
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `Length' on 1 arguments
...

and also this:

########> Diff in /home/travis/build/gap-packages/sla/gaproot/pkg/sla/tst/sla0\
5.tst:170
# Input is:
LeviMalcevDecomposition(K);
# Expected output:
[ <Lie algebra of dimension 3 over Rationals>, 
  <two-sided ideal in <Lie algebra of dimension 20 over Rationals>, 
      (dimension 17)> ]
# But found:
[ <Lie algebra of dimension 15 over Rationals>, 
  <Lie algebra of dimension 0 over Rationals> ]
########

If I may, I'd recommend to do all development via pull requests -- i.e. instead of directly pushing to master, push into a new remote branch and open a pull request for it. Then all tests are run for you, and if they passed, you can merge everything with a button click.

Alternatively, you can of course also work as before, but it might be useful to run the test suite before committing (e.g. via gap tst/testall.g from the package directory).