Introduce ZLIBNG_ENABLE_TESTS

Introduce ZLIBNG_ENABLE_TESTS

This patch adds the ability to run zlib-ng test suite against the
original zlib as follows:

    cmake -DZLIB_COMPAT=ON -DZLIBNG_ENABLE_TESTS=OFF .
    make
    LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu make test

The benefit of this is that modifications to the original zlib can be
tested with a more extensive zlib-ng's testsuite, and the assumptions
that the zlib-ng tests make can be validated against the original zlib.

In addition to a number of tests that exercise purely zlib-ng specific
API, there are a few that expect zlib-ng specific behavior from the
original zlib API:

- deflate() (obviously) emits different streams
- zlib-ng's deflatePrime() can take more than 16 bits
- zVersion() returns a different string

Adjust or disable the respective tests for ZLIBNG_ENABLE_TESTS=OFF.