Update tests/README.md

* h2 instead of h1 for sections
* add link to phpunit manual
* update setup instructions (now tests are in the main repository;) )
* Remove the suggestion to write Andrew to have write access to the repository
  (now tests are in the main repository:( )
State that phpunit must be run from tests dir
Fix sample output
Merge pull request #1529 from mlocati/fix-test-readme

Update tests/README.md