initialize variable in tests
initialize variable in tests

This was detected while running the tests with the `-Wconditional-uninitialized` flag

```
./autogen.sh
CC=clang CFLAGS="-Wconditional-uninitialized" ./configure
make check
```

The resulting warning is a false positive, but setting the value to -1
ensures that the CHECK below will fail if recid is never written to.

bitcoin-core/secp256k1,afk11/secp256k1,bitcoin-core/secp256k1,peterdettman/secp256k1,afk11/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,afk11/secp256k1,afk11/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,bitcoin/secp256k1,afk11/secp256k1,bitcoin/secp256k1