DT-1201: Run BLT tests with ORCA fixture

[danepowell]

• It would be nice not to have to hack the symlink, but I don't know how to avoid that: Absolute path repo URLs are made relative composer/composer#8700
• Update testing docs (against docs repo first).

Testing steps:

• Remove the vendor directory from the BLT working directory to avoid an autoloading issue: qa:automated-tests including broken vendor test cases orca#73
• Create the test fixture: /home/dane/packages/orca/bin/orca fixture:init -f --sut=acquia/blt --sut-only --core='~9' --dev
• Run the tests: ORCA_FIXTURE_DIR=/home/dane/packages/orca-build ./bin/orca qa:automated-tests --sut=acquia/blt --sut-only

[danepowell]

Ah... the problem now is that the Doctor (and possibly other components) expect MySQL and don't know what to do with SQLite.

[danepowell]

Blocked on an undisclosed breaking change in a Symfony Console patch release: #4066