This reverts commit 18f6ed50.

Thanks to Kilian Sprotte for reporting this.

  * Remove BT-SYSTEM package while preserving LOADability
    of .asd files.

  * Move test suite related bits from BT's TEST-OP to new
    BT-TEST's TEST-OP.

  * Make BT's TEST-OP simply invoke BT-TEST's TEST-OP.

Finished conversion from methods to functions with defaults (Stelian Ionescu <sionescu@common-lisp.net>)

Centralized loading and checking of impl-specific thread libraries (Stelian Ionescu <sionescu@common-lisp.net>)