Allow unit test DB config to be supplied via environment vars

[westonruter]

Based on our conversation here: https://twitter.com/jeremyfelt/status/707751217187819521

With this in place, you'd be able to brew install mysql, create the wordpress_unit_tests database:

CREATE DATABASE IF NOT EXISTS wordpress_unit_tests CHARACTER SET utf8;

And then add the following lines to your host machine's .bash_profile:

export WP_TESTS_DB_HOST=localhost
export WP_TESTS_DB_NAME=wordpress_unit_tests
export WP_TESTS_DB_USER=root
export WP_TESTS_DB_PASSWORD=''

Assuming you also have PHPUnit installed (brew install phpunit), you show now be able to run phpunit from a terminal on the host machine and execute the unit tests as fast as possible.

[westonruter]

@jeremyfelt in my testing, I found that running the entire test suite inside vagrant took around 3 minutes, whereas running the test suite completely from host machine took 2.5 minutes. Does this difference in performance align with what you've seen?

[jeremyfelt]

@westonruter Hmm. I think the WP core tests have gotten quite a bit quicker. The last time I ran them inside the VM, it took ~8 minutes while my local environment was around 3 minutes. Just now though, the VM tests took less than 2 minutes. I think I might be going crazy.

[westonruter]

I just published a writeup comparing running PHPUnit entirely in the VM, PHP on host but DB in VM, and then entirely on host without VM at all: https://make.xwp.co/2016/03/10/running-phpunit-tests-outside-vvv/