We do not support closing file descriptors automatically with
WordList. But (beside the fact that WordList instances require less
memory), we support reading a wordlist several times.

Make sure WordLists handle open file descriptors correctly.

WordLists should generally fullfill all promises of get_wordlist,
except closing file descriptors. The latter is not supported because
we might want to read a file several times.

We also support cryptographically signed wordlists with WordList instances.

It is better readable.

Make sure signed entries are stripped also if dash-escaped.

Create a single place where lines of wordlists can be refined,
regardless of their mode (crypt-signed or not).

We ignore empty lines also in signed wordlists. To check it, we need
an empty line in the sample.

Many wordlists have format "NNNNNN<tab>some-term". We try to
detect lines of this format and will parse them to get the term
(removing numbers and whitespaces).

and do not strip() if done already.

We need this regular expression to strip entries in wordlists like
the original 7776-terms wordlist from diceware.com.

As described in RFC 4880, dashes in wordlists might be escaped.
We have to take care of this when extracting wordlists.

In signed files, words with a starting dash are treated special
(they are "dash-escaped" as RFC 4880 says).

This sample wordlist contains only 2 entries but is
cryptographically signed. We use it to make sure we can also
handle signed wordlists.

The changed test could fail if the wordlist dir was relatively short.