Write unit tests checking that the right conditions are signaled.

[robert-strandh]

Provide unit tests that verify that the condition
EXPRESSION-CST-REQUIRED is signaled when RAW is called on some object
other than an EXPRESSION-CST.

Provide unit tests that verify that the condition CONS-CST-REQUIRED is
signaled when FIRST or REST is called on some object other than a
CONS-CST.