[Fix] Fix multi-node test tmp dir

[LXXXXR]

Hi,
/tmp dir is usually not shared therefore using a dir under current dir and clean it up afterwards is better.

Pls kindly take a look. Thank you.

Ref: open-mmlab/mmdetection#4017

[codecov]

Codecov Report

Merging #251 (d298ee6) into master (8c90a87) will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
+ Coverage   72.83%   72.86%   +0.02%     
==========================================
  Files          88       88              
  Lines        4455     4452       -3     
  Branches      782      780       -2     
==========================================
- Hits         3245     3244       -1     
+ Misses       1109     1108       -1     
+ Partials      101      100       -1     

Flag	Coverage Δ
unittests	72.86% <0.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcls/apis/test.py	23.72% <0.00%> (-0.21%)	⬇️
mmcls/models/builder.py	94.44% <0.00%> (+12.62%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8c90a87...d298ee6. Read the comment docs.