more robust test for subtree

From jhedstron in IRC conversation;

"jgraham909: for drush 6, i do want to move towards more unit tests in addition to the hash-based functional tests, so if you have time, feel free to write a unit test that explicitly checks directory structure and throw up a patch"

Please see attached patch that revises the subtree make process test.

Test run output below, now has 18 assertions around file structure.

jeff@ubuntu:/srv/www/vbdev/html/sites/all/modules/drush/tests$ git diff > ~/drush-subtree-unit-test.diff
jeff@ubuntu:/srv/www/vbdev/html/sites/all/modules/drush/tests$ phpunit --filter=testMakeSubtree
PHPUnit 3.6.10 by Sebastian Bergmann.

Configuration read from /srv/www/vbdev/html/sites/all/modules/drush/tests/phpunit.xml.dist

nivo-slider downloaded from https://github.com/downloads/gilbitron/Nivo-Slider/nivo-slider2.7.1.zip.                                      [ok]
fullcalendar downloaded from http://arshaw.com/fullcalendar/downloads/fullcalendar-1.5.3.zip.                                             [ok]
.
Log: Executing: /home/jeff/bin/drush --nocolor make --no-core /srv/www/vbdev/html/sites/all/modules/drush/tests/makefiles/subtree.make /tmp/drush-sandbox/subtree


Time: 2 seconds, Memory: 8.25Mb

OK (1 test, 19 assertions)