engine-key tests: make check_engine_keys.sh work with --enable-small

engine-key tests: make check_engine_keys.sh work with --enable-small

--enable-small eliminates one of the openssl errors the test is
looking for, so alter the grep also to account for the message in this
version.  Additionally output log.txt on failure so any test platform
gives an easy clue about what went wrong.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1592953354.2103.3.camel@HansenPartnership.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20102.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>