message: transport: Update README for test-fixture
diff: To run test http 0.9 programs (neqo-client and neqo-server):
  * `cargo build`
  -* `./target/debug/neqo-server 12345 -k key --db ./neqo-crypto/db`
  -* `./target/debug/neqo-client http://127.0.0.1:12345/ -o --db ./neqo-crypto/db`
  +* `./target/debug/neqo-server 12345 -k key --db ./test-fixture/db`
  +* `./target/debug/neqo-client http://127.0.0.1:12345/ -o --db ./test-fixture/db`
  To run test http 3 programs (neqo-client and neqo-http3-server):
  * `cargo build`
  -* `./target/debug/neqo-http3-server -p 12345 --db ./neqo-crypto/db`
  -* `./target/debug/neqo-client http://127.0.0.1:12345/ --db ./neqo-crypto/db`
  +* `./target/debug/neqo-http3-server -p 12345 --db ./test-fixture/db`
  +* `./target/debug/neqo-client http://127.0.0.1:12345/ --db ./test-fixture/db`
  