Don't depend on silverstripe/installer:phpunit.xml.dist for testing

[sminnee]

See silverstripe/silverstripe-installer#119

phpunit.xml.dist is the test configuration provided for new sites. If cow needs to tests a number of modules before bundling the releases, it should rely on another configuration source.

This could be ~/.cow/phpunit.xml.dist,phpunit.xml.dist.cow, or something else (e.g. pre-release-build-test setting in composer.json), but we can't break project developer experience in order to support our build tooling.

[sminnee]

A pre-release-build-test setting in composer.json would probably be my preferences. The scripts section should be useful for this.

[tractorcow]

That would be good, because different branches of installer could have custom release steps, so it fits that these live in the code base itself.