Add tests for Archive_Tar

Drupal 8/9 now include Archive_Tar via composer.

Drupal 7 still has its own copy of most of the code from the third party library in system.tar.inc in the system module.

There's very little if any test coverage of Archive_Tar within core.

I'm not sure about D8/9 but for D7 at least it'd be useful to have some basic integration tests which ensure that when updates to the library are made, we don't introduce regressions in functionality.

FWIW when manually testing the latest updates, I did something like this:

/path/to/drupal-7.x$ wget -P /tmp https://ftp.drupal.org/files/projects/seckit-7.x-1.11.tar.gz

/path/to/drupal-7.x$ drush php
Psy Shell v0.9.9 (PHP 7.2.24-0ubuntu0.18.04.1 — cli) by Justin Hileman
>>> $tar = new Archive_Tar('/tmp/seckit-7.x-1.11.tar.gz');
=> Archive_Tar {#5914
     +_tarname: "/tmp/seckit-7.x-1.11.tar.gz",
     +_compress: true,
     +_compress_type: "gz",
     +_separator: " ",
     +_file: 0,
     +_temp_tarname: "",
     +_ignore_regexp: "",
     +error_object: null,
     +"_fmt": "Z100filename/Z8mode/Z8uid/Z8gid/Z12size/Z12mtime/Z8checksum/Z1typeflag/Z100link/Z6magic/Z2version/Z32uname/Z32gname/Z8devmajor/Z8devminor/Z131prefix",
   }

>>> $tar->listContent();
=> [
     [
       "checksum" => 4636,
       "filename" => "seckit/",
       "mode" => 493,
       "uid" => 0,
       "gid" => 0,
       "size" => 0,
       "mtime" => 1561463898,
       "typeflag" => "5",
       "link" => "",
     ],
     [
       "checksum" => 5694,
       "filename" => "seckit/CHANGELOG.txt",

...etc...

...in order to ensure that the basic functionality of the class worked.