Run different unit tests according to the changed files

[mhaoli]

Currently, travis runs all unit tests even if we only modify doc files. We should not run unit tests when only doc files are modified. I wonder whether there is any way to achieve this.

Or would it be better if travis can choose to run some unit tests according to the changed files in this PR?

[terrytangyuan]

Yes, we should skip tests if there’s no code change.