Better case-lambda tests

[dpk]

The problem with the original case-lambda tests is that they could
actually pass if the + and * procedures were implemented correctly
but case-lambda itself wasn’t.

Specifically, an attempted optimized case-lambda implementation
which looked at the length of its arguments list and subsequently
erroneously always chose the variadic clause of the plus procedure
would still pass the test, because plus in this case recreated the
behaviour of the + procedure used for the test; it was never
actually observable whether the args clause or one of the more
specific clauses had been used to generate the result. Similar applies
to the mult test: although in that case an implementation could only
have erroneously chosen the (x y . z) clause in the two-argument
case, it would still have been an error invisible to the test cases.

I’ve also added a test which attempts to ensure that a redundant
clause will never match. This may cause a warning on Schemes which
detect such clauses at compile time, but R7RS does not explicitly
define such case-lambda expressions as erroneous in any way, so it
would be wrong (and non-conformant) for it to stop the tests running
altogether.

(This patch mainly useful because Chibi’s R7RS tests are sometimes
used by other implementations to ensure conformance. Chibi passed
these tests in any case.)