Added test file for standalone completion

test/vader/complete_standalone.vader

@@ -0,0 +1,25 @@
+Before:
+  call delete('/tmp/vader', 'rf')
+  call mkdir('/tmp/vader')
+  call system('touch /tmp/vader/fig_1.tex')
+  call system('touch /tmp/vader/fig_2.tex')
+  call system('touch /tmp/vader/fig_3.tex')
+  w /tmp/vader/test.tex
+  silent e /tmp/vader/test.tex
+  setf tex
+
+After:
+  w
+  bd! /tmp/vader/test.tex
+
+Given tex:
+  \includestandalone{
+
+Do:
+  A\<c-x>\<c-o>\<c-n>}
+
+Then:
+  AssertEqual
+    \ '\includestandalone{fig_2}',
+    \ readfile('/tmp/vader/test.tex')[-1]
+