Changeset faf9e19 for libcfa/prelude

Explicate the present state for test-suite programs being free of warnings.

Now, test-suite compilation occurs in either strict or lax "wflags" mode. Strict mode is equivalent to what used to be done for the nowarn folder, lax to the rest of the test suite. Makefile.am now owns a list of tests that opt for lax wflags. About 70% of the tests opt for lax.

Remove special handling for the nowarn folder. Now, it simply does not appear on the opt-for-lax list.

Remove former tests

nowarn/fstream
nowarn/list
nowarn/string

because, respectively

io/io
list/dlist-insert-remove
collections/string-api-coverage

obviate them by getting strict wflags.