bug(c++,spark): clean the output directory when generating data in unit test

### Describe the bug, including details regarding any error messages, version, and platform.

In c++ or spark unit test, we usually creating the graphar data to `/tmp` dir and may check the generated file num with an `assert`. But the c++ and spark unit test may generate useless files for each other and make the assertion failed.

I suggest we can clean the output directory before write out the files in unit test of c++ and spark.

### Solution

As Sem suggested, we can making the clean operation as a part of the top-level `make clean` of C++/Spark library.

### Component(s)

C++, Spark