Add a simple method to run regression tests

This is only useful when building shared libraries.  This allows us to
run our tests against newer libraries when the time comes.  Simply do
this:

    OPENSSL_REGRESSION=/other/OpenSSL/build/tree make test

($OPENSSL_REGRESSION *must* be an absolute path)
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5619)

util/shlib_wrap.sh.in

 #!/bin/sh
 
+# To test this OpenSSL version's applications against another version's
+# shared libraries, simply set
+#
+#     OPENSSL_REGRESSION=/path/to/other/OpenSSL/build/tree
+if [ -n "$OPENSSL_REGRESSION" ]; then
+    shlibwrap="$OPENSSL_REGRESSION/util/shlib_wrap.sh"
+    if [ -x "$shlibwrap" ]; then
+        # We clear OPENSSL_REGRESSION to avoid a loop, should the shlib_wrap.sh
+        # we exec also support that mechanism...
+        OPENSSL_REGRESSION= exec "$shlibwrap" "$@"
+    else
+        if [ -f "$shlibwrap" ]; then
+            echo "Not permitted to run $shlibwrap" >&2
+        else
+            echo "No $shlibwrap, perhaps OPENSSL_REGRESSION isn't properly set?" >&2
+        fi
+        exit 1
+    fi
+fi
+
 [ $# -ne 0 ] || set -x		# debug mode without arguments:-)
 
 THERE="`echo $0 | sed -e 's|[^/]*$||' 2>/dev/null`.."