test/context_internal_test.c: don't initialize as a separate test

Because test order can be randomized, running foo_init() as a separate
test is unsafe practice.  Instead, we make it possible to call it
multiple times, and call it at the start of each separate test.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8288)