introduce and use SECP256K1_{FE,GE,GEJ}_VERIFY macros

By providing an uppercase variant of these verification functions, it is
better visible that it is test code and surrounding `#ifdef VERIFY`
blocks can be removed (if there is no other code around that could
remain in production mode), as they don't serve their purpose any more.

At some places intentional blank lines are inserted for grouping and
better readadbility.

src/modules/ellswift/tests_impl.h

@@ -285,7 +285,7 @@ void run_ellswift_tests(void) {
         ret = secp256k1_ellswift_xdh(CTX, share32, ell64, ell64, sec32, i & 1, &ellswift_xdh_hash_x32, NULL);
         CHECK(ret);
         (void)secp256k1_fe_set_b32_limit(&share_x, share32); /* no overflow is possible */
-        secp256k1_fe_verify(&share_x);
+        SECP256K1_FE_VERIFY(&share_x);
         /* Compute seckey*pubkey directly. */
         secp256k1_ecmult(&resj, &decj, &sec, NULL);
         secp256k1_ge_set_gej(&res, &resj);