test-help.t: add test for 'hg help extensions'

This demonstrates the defect that deprecated extensions show up in the
output. A followup patch will fix this defect.

--HG--
extra : rebase_source : 2035ea7e75f090fd87c9ae194f46760c479a6cc3
help: exclude deprecated extensions in the disabled part of 'help extensions'

--HG--
extra : rebase_source : 31104e2b4e38452cf41771ce704665900d1ea95a
merge with crew