fix(checkhealth): skip vim.health #18816

Problem:
https://github.com/neovim/neovim/pull/18720#issuecomment-1142614996

The vim.health module is detected as a healthcheck, which produces spurious errors:

    vim: require("vim.health").check()
    ========================================================================
    - ERROR: Failed to run healthcheck for "vim" plugin. Exception:
      function health#check, line 20
      Vim(eval):E5108: Error executing lua [string "luaeval()"]:1: attempt to call field 'check' (a nil value)
      stack traceback:
      [string "luaeval()"]:1: in main chunk

Solution:
Skip vim.health when discovering healthchecks.

test/functional/plugin/health_spec.lua

@@ -5,7 +5,7 @@ local Screen = require('test.functional.ui.screen')
 local clear = helpers.clear
 local curbuf_contents = helpers.curbuf_contents
 local command = helpers.command
-local eq = helpers.eq
+local eq, neq = helpers.eq, helpers.neq
 local getcompletion = helpers.funcs.getcompletion
 
 describe(':checkhealth', function()
@@ -37,6 +37,7 @@ describe(':checkhealth', function()
     eq('nvim', getcompletion('nvim', 'checkhealth')[1])
     eq('provider', getcompletion('prov', 'checkhealth')[1])
     eq('vim.lsp', getcompletion('vim.ls', 'checkhealth')[1])
+    neq('vim', getcompletion('^vim', 'checkhealth')[1])  -- should not complete vim.health
   end)
 end)
 
@@ -242,6 +243,13 @@ describe('health.vim', function()
           - ERROR: No healthcheck found for "non_existent_healthcheck" plugin.
         ]])
     end)
+
+    it("does not use vim.health as a healtcheck", function()
+      -- vim.health is not a healthcheck
+      command("checkhealth vim")
+      helpers.expect([[
+      ERROR: No healthchecks found.]])
+    end)
   end)
 end)