test: mark non-actionable tests as N/A #40207

Problem:
Listing individual "skipped" notices for each non-actionable or
NVIM_TEST_INTEG test is noisy and makes the "skipped" list overwhelming
and less meaningful.

Solution:
- Mark non-actionable tests with "N/A".
- Report NVIM_TEST_INTEG tests as a separate 1-line summary.

test/functional/vimscript/eval_spec.lua

@@ -93,7 +93,7 @@ describe('backtick expansion', function()
   end)
 
   it('with shell=fish', function()
-    t.skip(fn.executable('fish') == 0, 'missing "fish" command')
+    t.skip(fn.executable('fish') == 0, 'N/A: missing "fish" command')
 
     command('set shell=fish')
     command(':silent args `echo ***2`')