Fix help test in dev shell

Not sure what the intent was expecting help.sh to fail in the main suite, but it caused `meson test` to fail inside a `nix develop` shell:

  $ meson test help --print-errorlogs
  ninja: Entering directory `/home/eelco/Dev/nix-master/build'
  1/1 nix-functional-tests:main / help        UNEXPECTEDPASS   4.02s

tests/functional/help.sh

@@ -2,6 +2,8 @@
 
 source common.sh
 
+[[ $(type -p man) ]] || skipTest "'man' not installed"
+
 # test help output
 
 nix-build --help

tests/functional/meson.build

@@ -242,8 +242,6 @@ foreach suite : suites
       # Used for target dependency/ordering tracking, not adding compiler flags or anything.
       depends : suite['deps'],
       workdir : workdir,
-      # Won't pass until man pages are generated
-      should_fail : suite['name'] == 'main' and script == 'help.sh'
     )
   endforeach
 endforeach