The SIGNALS check can correctly detect whether a block of code signals
a warning, but it also aborts the code block's execution. That's
appropriate for checking for an error, but not for a warning. This new
check confirms that a warning of a particular class is raised, a la
SIGNALS, but also allows the block of code in its scope to complete,
which may be more natural for some cases.

SIGNALS now captures the return value of the signaling block, EXCEPT
when the condition signaled is an error, in which case there may not
be a value to return.

Changes in tests.lisp illustrate the trade-offs.