test-lib: Allow completions containing whitespace.

This is necessary to test the completion of task text.

tests/README

@@ -191,11 +191,13 @@ library for your script to use.
    This takes three strings as parameter.  Based on <cmdline>,
    the todo_completion script is triggered in the current test
    environment and completions are compared with <completions>,
-   which should be a space-separated list.  Include a trailing
-   space in <cmdline> when you want to check new argument
-   completion; otherwise, completion is triggered with the
-   context of the last argument.  <message> should state what it
-   is testing.
+   which should be a space-separated list.  If any completion
+   contains whitespace, quote it; the entire <completions>
+   argument is eval()'ed.
+   Include a trailing space in <cmdline> when you want to check
+   new argument completion; otherwise, completion is triggered
+   with the context of the last argument. <message> should state
+   what it is testing.
 
  - test_todo_custom_completion <completefunc> <message> <cmdline> <completions>