Relax checks for extended capability to support new format (#5)

In the 20180331 release, the format was slightly changed:

> 20180331
>         + improve terminfo write/read by modifying the fourth item of the
>           extended header to denote the number of valid strings in the extended
>           string table (prompted by a comment in unibilium's sources).

Since the number of valid string capabilities is not necessarily the
same as extstrslen, it's not possible to sanity check the total number
of items up front anymore.

Include a new test with an updated screen terminfo dump.