Break into the debugger upon error

[foretspaisibles]

This PR allows to open a debugger when unexpected errors occur in the testsuite.

Use a call to (testcase-break-into-the-debugger-on-errors) to arrange for this to happen. When this configuration is active, the following restarts are available in the debugger:
• ASSERTION-RETRY Which retries to evaluate the arguments of the failing assertion
and the assertion itself.
• TESTCASE-RETRY Which retries the current testcase.
• TESTCASE-CONTINUE Which registers an error and continues the current testcase.
• TESTCASE-RETURN Which registers an error and immediately returns from the current testcase.
• TESTCASE-STEP-UPWhichregistersanerrorandcontinuethecurrenttestcasewith- out breaking into the debugger.
• TESTCASE-SCROLL Which registers an error and continue the current testcase and other testcases higher in the call stack without breaking into the debugger.

The testsuites a-failing-argument-testsuite, a-failing-testcase-testsuite and a-compound-failing-testsuite can be used to experiment with this as in:

(ql:quickload '#:org.melusina.confidence/user)
(in-package #:org.melusina.confidence/user)
(confidence:testcase-break-into-the-debugger-on-errors)
(testsuite:a-compound-failing-testsuite)

This solves #4 opened by @lukego