Add some behavioural tests

[asgrim]

At this stage, it would be worth adding some behavioural tests to model the PIE command at a high level. This would allow us to refactor internals without breaking critical functionality.

Behaviour to cover:

• Downloading a package using various versioning/stability definitions
• Downloading a package using a different target PHP install (--with-php-config / --with-php-path)
• Building a package
• Building a package using a different target PHP install (--with-php-config / --with-php-path)
• Building a package and specifying configure options
• Installing a package
• Installing a package using a different target PHP install (--with-php-config / --with-php-path)
• Installing a package and specifying configure options
• Installing a package with mismatching ZTS/NTS should fail
• Installing a package should notify Packagist
• Installing a package in an unprivileged environment (i.e. need sudo)

Naturally, these need to run in CI too.