tests: if server failed to start, print exit code instead of timing out

```
% cargo +stable test --lib ssl::test::test_connect_with_alpn_successful_single_match --features=v102
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running /Users/nga/devel/left/rust-openssl/target/debug/deps/openssl-a38e12a3527f6932

running 1 test
test ssl::test::test_connect_with_alpn_successful_single_match ... FAILED

failures:

---- ssl::test::test_connect_with_alpn_successful_single_match stdout ----
	thread 'ssl::test::test_connect_with_alpn_successful_single_match' panicked at 'server exited: exit code: 1', src/ssl/test.rs:91:24
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    ssl::test::test_connect_with_alpn_successful_single_match

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 159 filtered out
```