legacy tests: migrate test18

src/nvim/testdir/test18.in

@@ -1,16 +0,0 @@
-Tests for not doing smart indenting when it isn't set.
-
-STARTTEST
-:so small.vim
-:set nocin nosi ai
-/some
-2cc#test
-:?start?,$w! test.out
-:qa!
-ENDTEST
-
-start text
-		some test text
-		test text
-test text
-		test text

src/nvim/testdir/test18.ok

@@ -1,4 +0,0 @@
-start text
-		#test
-test text
-		test text

test/functional/legacy/018_unset_smart_indenting_spec.lua

@@ -0,0 +1,28 @@
+-- Tests for not doing smart indenting when it isn't set.
+
+local helpers = require('test.functional.helpers')
+local clear, feed, insert = helpers.clear, helpers.feed, helpers.insert
+local execute, expect = helpers.execute, helpers.expect
+
+describe('unset smart indenting', function()
+  setup(clear)
+
+  it('is working', function()
+    insert([[
+      start text
+              some test text
+              test text
+      test text
+              test text]])
+
+    execute('set nocin nosi ai')
+    execute('/some')
+    feed('2cc#test<Esc>')
+
+    expect([[
+      start text
+              #test
+      test text
+              test text]])
+  end)
+end)