test: driver: fix -Wunused-but-set-variable

test: driver: fix -Wunused-but-set-variable

The value of 'num_failed_inner' isn't ever used.

Fixes this error with Clang 15:
```
test/testutil/driver.c:341:17: error: variable 'num_failed_inner' set but not used [-Werror,-Wunused-but-set-variable]
            int num_failed_inner = 0;
                ^
1 error generated.
```

Signed-off-by: Sam James <sam@gentoo.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19450)