testing: update "pushlog compatible" hg version in test-mozext.t

`$PUSHLOGHG` is a variable that points to a pushlog-compatible
version of Mercurial in the test suite for use. Update to the
actual version in use since we removed the previous version
from the test environment.

--HG--
extra : rebase_source : 9d5d5ff679501351815df61a209d01f9b55432bb

hgext/mozext/tests/test-mozext.t

@@ -11,7 +11,7 @@
   > writeuri = ssh://user@dummy/$TESTTMP/
   > EOF
 
-  $ export PUSHLOGHG=/app/venv/mercurials/5.3.2/bin/hg
+  $ export PUSHLOGHG=/app/venv/mercurials/5.9.3/bin/hg
 
   $ export USER=hguser
   $ hg init mozilla-central