testsuite: provide the testing component as a library
testsuite: provide the testing component as a library

A few tests rely on a Testing module located in testsuite/lib.

The legacy testing infrastructure links it into the programs that
use it as a module, but in the context of ocamltest it seems better
to provide testing as a library.

This commit thus modifies the build system of the testsuite to
also build testing as a library, both in bytecode and native formats.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml