Add main() test methods to reduce test boilerplate.

Simple tests only need to implement register_tests().
Tests that need a custom main() should implement test_main(). This will
be wrapped in a main() that performs common setup/teardown (currently
crypto-mdebug).

Note that for normal development, enable-asan is usually
sufficient for detecting leaks, and more versatile.

enable-crypto-mdebug is stricter as it will also
insist that all static variables be freed. This is useful for debugging
library init/deinit; however, it also means that test_main() must free
everything it allocates.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>