tests: fix Test_help_tagjump()

The Vim version of Test_help_tagjump() tests for `:help sm?le` here. That
command got removed from Nvim, so the test was changed to check against `:help
sp?it` instead.

The new test already handled the case that on Win `:h split` would jump to the
entry for split() and on all other systems to the entry for :split.

Then this commit happened:

   bb3aa824b lua/stdlib: vim.inspect, string functions

Since then `:h split` would jump to split() for macOS as well! I'm not sure why.

Anyway, instead of adding another check for has('mac'), we change the test once
more to be more akin to the original test. Instead of testing for :smile, which
is exclusive to Vim, we check against :checkhealth, which is exclusive to Nvim.

src/nvim/testdir/test_help_tagjump.vim

@@ -28,9 +28,9 @@ func Test_help_tagjump()
   call assert_true(getline('.') =~ '\*quotestar\*')
   helpclose
 
-  help sp?it
+  help ch?ckhealth
   call assert_equal("help", &filetype)
-  call assert_true(getline('.') =~ '\*'.(has('win32') ? 'split()' : ':split').'\*')
+  call assert_true(getline('.') =~ '\*:checkhealth\*')
   helpclose
 
   help :?