bind server address once

[estin]

Hello!
It's me again.

In my test all mocks are unique and I want run tests without --test-threads=1

$ cargo test --all
...
running 3 tests
thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 98, message: "Address already in use" } }', /checkout/src/libcore/result.rs:860:4
note: Run with `RUST_BACKTRACE=1` for a backtrace.
thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 98, message: "Address already in use" } }', /checkout/src/libcore/result.rs:860:4
test test_1... ok
test test_2 ... ok
test test_3 ... ok

With this PR tests runs without noise about paniced threads.

$ cargo test --all
...
running 3 tests                    
test test_1 ... ok                             
test test_2 ... ok       
test test_3 ... ok       

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out 

Note this PR does't solve problem testings when mocks are duplicated without --test-threads=1

Sorry about my poor English.

[lipanski]

0.8.2 is out. thanks 🎉