* NLTK support: Update test to use multiple corpora

So that the incorrect handling of multiple IDs seen in #444 would
have been caught.

Also switches to some of the smaller corpora, to reduce time spent
downloading during tests (see sizes on http://www.nltk.org/nltk_data/).

* NLTK support: Fix passing of multiple corpora identifiers

As part of fixing the shellcheck warnigns in #438, double quotes had
been placed around `$nltk_packages` passed to the `nltk.downloader`,
which causes multiple identifiers to be treated as though it were just
one identifier that contains spaces.

The docs for the shellcheck warning in question recommend using arrays
if the intended behaviour really is to split on spaces:
https://github.com/koalaman/shellcheck/wiki/SC2086#exceptions

As such, `readarray` has been used, which is present in bash >=4.
The `[*]` array form is used in the log message, to prevent shellcheck
warning SC2145, whereas `[@]` is used when passed to `nltk.downloader`
to ensure the array elements are unpacked as required.

Note: Both before and after this fix, using anything but unix line
endings in `nltk.txt` will also cause breakage.