Avoid getting unresolved referense to bn_expand2 in test/bntest.c

Issue identified on Solaris by Erik Forsberg <erik@efca.com>
Reviewed-by: NRich Salz <rsalz@openssl.org>

test/bntest.c

@@ -80,6 +80,19 @@
 #include <openssl/x509.h>
 #include <openssl/err.h>
 
+/*
+ * In bn_lcl.h, bn_expand() is defined as a static ossl_inline function.
+ * This is fine in itself, it will end up as an unused static function in
+ * the worst case.  However, it referenses bn_expand2(), which is a private
+ * function in libcrypto and therefore unavailable on some systems.  This
+ * may result in a linker error because of unresolved symbols.
+ *
+ * To avoid this, we define a dummy variant of bn_expand2() here, and to
+ * avoid possible clashes with libcrypto, we rename it first, using a macro.
+ */
+#define bn_expand2 dummy_bn_expand2
+static BIGNUM *bn_expand2(BIGNUM *b, int words) { return NULL; }
+
 #include "../crypto/bn/bn_lcl.h"
 
 static const int num0 = 100;           /* number of tests */