Build cleanup: Remove the VMS hack from test/build.info

There was a hack specifically for VMS, which involved setting a make
variable to indicate that test/libtestutil contains a 'main'.

Instead, we use the new attributes 'has_main' to indicate this, and
let the VMS build file template fend with it appropriately.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8125)