Add test for NimUsages

tests/tests/vim/usages.vader

@@ -0,0 +1,25 @@
+Given nim:
+  proc print(str: string) = echo str
+  echo "this"
+  print "test"
+  echo "is"
+  print "very"
+  echo "nice"
+
+Do:
+  :saveas! vader_workbench.nim\<cr>
+  ggw
+  :NimUsages\<cr>
+  G
+
+Then:
+  AssertEqual line("."), 3
+
+Do:
+  :cclose\<cr>
+  gg$bb
+  :NimUsages\<cr>
+  G
+
+Then:
+  AssertEqual line("."), 4