Use `t.TempDir()` in unit test

[squakez]

We are making use of temporary files in test which create a large amount of useless data. We should leverage the t.TempDir() to let the Golang testing framework to clean any temporary data when the test is complete.

[michalvavrik]

I'll have a look this week.

[michalvavrik]

Should be done here #6251.