test-help: improve test coverage

Adds a test for displaying a command list with a command with no help text.

Extends test coverage of commands.help_ by 1 line.

tests/test-help

@@ -51,4 +51,7 b' hg help nohelp'
 echo %% test that default list of commands omits extension commands
 hg help
 
+echo %% test list of commands with command with no help text
+hg help helpext
+
 exit 0

tests/test-help.out

@@ -542,3 +542,11 b' additional help topics:'
  extensions   Using additional features
 
 use "hg -v help" to show aliases and global options
+%% test list of commands with command with no help text
+helpext extension - no help text available
+
+list of commands:
+
+ nohelp   (no help text available)
+
+use "hg -v help helpext" to show aliases and global options