tests: make zstd-related output optional
Caught by the buildbot with --pure, fix suggested by indygreg.
--- a/tests/test-help.t Mon Apr 03 18:58:00 2017 -0400
+++ b/tests/test-help.t Mon Apr 03 19:45:09 2017 -0400
@@ -1848,7 +1848,7 @@
An algorithm that produces smaller bundles than "gzip".
This engine will likely produce smaller bundles than "gzip" but will be
"gzip"
- better compression than "gzip". It also frequently yields better
+ better compression than "gzip". It also frequently yields better (?)
Test usage of section marks in help documents