test: fnamemodify with :8 filename modifier

test/functional/eval/fnamemodify_spec.lua

@@ -4,10 +4,19 @@ local eq = helpers.eq
 local iswin = helpers.iswin
 local fnamemodify = helpers.funcs.fnamemodify
 local command = helpers.command
+local write_file = helpers.write_file
 
 describe('fnamemodify()', function()
+  setup(function()
+    write_file('Xtest-fnamemodify.txt', [[foobar]])
+  end)
+
   before_each(clear)
 
+  teardown(function()
+    os.remove('Xtest-fnamemodify.txt')
+  end)
+
   it('works', function()
     if iswin() then
       local drive_f = io.popen('for %P in (%CD%) do @echo %~dP', 'r')
@@ -29,4 +38,8 @@ describe('fnamemodify()', function()
       eq('/', fnamemodify([[/]], ':p'))
     end
   end)
+
+  it(':8 works', function()
+    eq('Xtest-fnamemodify.txt', fnamemodify([[Xtest-fnamemodify.txt]], ':8'))
+  end)
 end)