diff -r 0760282995cf -r 7289eac777ec tests/test-hook.t
--- a/tests/test-hook.t	Wed Feb 10 22:43:16 2021 +0100
+++ b/tests/test-hook.t	Wed Feb 10 23:21:21 2021 +0100
@@ -1407,13 +1407,21 @@
 
   $ cat << EOF >> .hg/hgrc
   > [hooks]
-  > pre-version.testing-default=echo '### default ###' plain: \$HGPLAIN
+  > pre-version.testing-default=echo '### default ###' plain: \${HGPLAIN:-'<unset>'}
+  > pre-version.testing-yes=echo '### yes #######' plain: \${HGPLAIN:-'<unset>'}
+  > pre-version.testing-yes:run-with-plain=yes
+  > pre-version.testing-no=echo '### no ########' plain: \${HGPLAIN:-'<unset>'}
+  > pre-version.testing-no:run-with-plain=no
   > EOF
 
   $ (unset HGPLAIN; hg version --quiet)
   ### default ### plain: 1
+  ### yes ####### plain: 1
+  ### no ######## plain: <unset>
   Mercurial Distributed SCM (*) (glob)
 
   $ HGPLAIN=1 hg version --quiet
   ### default ### plain: 1
+  ### yes ####### plain: 1
+  ### no ######## plain: <unset>
   Mercurial Distributed SCM (*) (glob)