Added instructions for running subsets of tests in Getting Started guide
Added instructions for running subsets of tests in Getting Started guide

Information in the blog post at
https://reviewboardstudents.wordpress.com/2011/09/24/running-subsets-of-review-board-tests/
has been added to the codebase Getting Started guide in the Running Unit Tests section. The
guide explains how to run sebset of Review Board unit tests.

Testing Done:
Ran 'make html' and checked the .html to make sure it links properly.

Reviewed at https://reviews.reviewboard.org/r/6770/

KnowNo/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,davidt/reviewboard,custode/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,custode/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,chipx86/reviewboard,brennie/reviewboard,beol/reviewboard,custode/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,KnowNo/reviewboard,davidt/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,beol/reviewboard,beol/reviewboard,davidt/reviewboard,KnowNo/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,davidt/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,custode/reviewboard,beol/reviewboard