Remove tests dependence on e_os.h

Apart from ssltest_old.c, the test suite relied on e_os.h for the
OSSL_NELEM macro and nothing else.

The ssltest_old.c also requires EXIT and some socket macros.

Create a new header to define the OSSL_NELEM macro and use that instead.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4186)