unified build scheme: adjust test framework for out of source build tree

To be able to run tests when we've built in a directory other than
the source tree, the testing framework needs a few adjustments.

test/testlib/OpenSSL/Test.pm needs to know where it can find
shlib_wrap.sh, and a number of other tests need to be told a different
place to find engines than what they may be able to figure out on
their own.  Relying to $TOP is not enough, $SRCTOP and $BLDTOP can be
used as an alternative.

As part of this change, top_file and top_dir are removed and
srctop_file, bldtop_file, srctop_dir and bldtop_dir take their place.

Reviewed-by: Ben Laurie <ben@openssl.org>