tests/wolfmath: size sp_gcd dest by b->used, not a hard-coded 2

test_wc_SpIntExptGcdDecisionCoverage sized r with sp_init_size(&r, 2)
expecting sp_gcd(2^140, 2^70, &r) == 0, on the assumption that 2^70
occupies 2 digits. That only holds for 64-bit SP_WORD_SIZE; on a 32-bit
build (e.g. --enable-sp-math-all with ALT_ECC_SIZE under -m32) 2^70 spans
3 digits, so r->size(2) < b->used(3) trips sp_gcd's dest-size check and it
returns MP_VAL. Size r to b.used so the "r->size < b->used" operand stays
false (the intended MC/DC pair) for any word size.

Author: danielinux

tests/api/test_wolfmath.c

@@ -946,8 +946,10 @@ int test_wc_SpIntExptGcdDecisionCoverage(void)
     ExpectIntEQ(sp_gcd(&a, &b, &r), WC_NO_ERR_TRACE(MP_VAL));
     /* Same a > b shape, but r sized to cover b->used exactly: the second
      * clause's r->size < b->used half now goes false while a->used <
-     * b->used stays true, isolating that operand's independence pair. */
-    ExpectIntEQ(sp_init_size(&r, 2), 0);
+     * b->used stays true, isolating that operand's independence pair.
+     * Size r to b->used (not a hard-coded 2) so this holds regardless of
+     * SP_WORD_SIZE - at 32-bit words 2^70 occupies 3 digits, not 2. */
+    ExpectIntEQ(sp_init_size(&r, b.used), 0);
     ExpectIntEQ(sp_gcd(&a, &b, &r), 0);
     ExpectIntEQ(sp_init(&r), 0);
     ExpectIntEQ(sp_gcd(&a, &b, &r), 0);