Make 'tmpdir' public to be cleanable with using Test::TCP

I actively use that cool module with Test::TCP for redis-based development. Really cool! But I found, that it pollutes /tmp directory when I intensively launch tests :) 

```
ind /tmp -name 'redis.conf'
/tmp/xcsfr8NZN0/redis.conf
/tmp/4dR6McYzSz/redis.conf
/tmp/acmpLkcnYG/redis.conf
/tmp/OmU9VBIyTg/redis.conf
/tmp/4UTt0Y3k0h/redis.conf
/tmp/nOOo1d_UVB/redis.conf
/tmp/di_Z1PM4qg/redis.conf
/tmp/nfiItJLInr/redis.conf
/tmp/mvuCL81Jel/redis.conf
/tmp/AaLF2tI9Mu/redis.conf
/tmp/PjnV6ykbr5/redis.conf
/tmp/V1he6CpGvV/redis.conf
/tmp/mYnivSa3Qk/redis.conf
/tmp/r3JtNq5HZg/redis.conf
/tmp/a8r82o3ydm/redis.conf
/tmp/LetdAX5E_r/redis.conf
/tmp/_9GLBISdq2/redis.conf
/tmp/jy_WiDNilM/redis.conf
/tmp/JWrPSxfIu6/redis.conf
/tmp/DSfp4jbcNx/redis.conf
/tmp/mWaTylBEGp/redis.conf
/tmp/bAxtyOQjVw/redis.conf
/tmp/XUCs8re7Kk/redis.conf
/tmp/A7gkJjjKl7/redis.conf
/tmp/8bTmi0lrx4/redis.conf
/tmp/GWTF3GrhWz/redis.conf
/tmp/R7nwrHcmD5/redis.conf
/tmp/02r6i1GHNu/redis.conf
/tmp/rM_WsR0lkf/redis.conf
/tmp/m3_LZK9jhv/redis.conf
/tmp/0I36ZNJT7w/redis.conf
/tmp/2WfStvW0la/redis.conf
/tmp/LNYqaHDBim/redis.conf
/tmp/QnmKX_cF44/redis.conf
/tmp/eZCp1teB79/redis.conf
/tmp/eCCl7BrFQF/redis.conf
/tmp/jKGUmi1Y0q/redis.conf
/tmp/rzhOekwVMl/redis.conf
/tmp/_rZ6QN2u4m/redis.conf
/tmp/LpDCamq8W6/redis.conf
/tmp/EAuEIFnSfN/redis.conf
/tmp/GC34z10doc/redis.conf
/tmp/6ciYVBmpoJ/redis.conf
/tmp/9C2o8vxy1t/redis.conf
/tmp/8_YTZ5hgjm/redis.conf
/tmp/ZGTIxNvpe5/redis.conf
/tmp/ZYhIJnCFN1/redis.conf
/tmp/o9ogKVxKPF/redis.conf
/tmp/IShmCiQwmv/redis.conf
/tmp/Mont0sbtGa/redis.conf
/tmp/yxGJpDpba9/redis.conf
/tmp/UniEfvvEpV/redis.conf
/tmp/m7Ktdm0MUF/redis.conf
/tmp/0U4f9X9OpK/redis.conf
/tmp/4gV3AlQrFk/redis.conf
/tmp/lz7qv9rnbQ/redis.conf
/tmp/boZiI8m2Z6/redis.conf
/tmp/LRm6aRh8WF/redis.conf
/tmp/2TkOagGzj1/redis.conf
/tmp/LA5ExupmDw/redis.conf
```

I've made tmpdir attribute public, documented it and written an test for it.