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/lua/system_spec.lua

@@ -175,8 +175,8 @@ describe('vim.system', function()
   end
 
   it('always captures all content of stdout/stderr #30846', function()
-    t.skip(n.fn.executable('git') == 0, 'missing "git" command')
-    t.skip(n.fn.isdirectory('.git') == 0, 'missing ".git" directory')
+    t.skip(n.fn.executable('git') == 0, 'N/A: missing "git" command')
+    t.skip(n.fn.isdirectory('.git') == 0, 'N/A: missing ".git" directory')
     eq(
       0,
       exec_lua(function()