Silence two style warnings from the test harness.

Silence two style warnings from the test harness.

Currently running any test emits a warning that REALLY-INVOKE-DEBUGGER
is undefined. Running any test that uses IMPURE-RUNNER additionally
emits a warning that *BREAK-ON-ERROR*'s lexical binding is used despite
its name indicating specialness.

Silence these two by moving the definition of REALLY-INVOKE-DEBUGGER
before its use and by adding a SPECIAL declaration for *BREAK-ON-ERROR*
in RUN-IMPURE-IN-CHILD-SBCL in a form that is executed in the child
SBCL.