Format and clean test files

The following discussion from !106 (merged) should be addressed:

• @vzavadskyi started a discussion: (+1 comment)

  @maxfl thank you for MR. Questions:

  • Some tests have a numerical identifier in the end of the name. Like 00 and 01. However, what is agreement on the numbering of tests? 19 tests have testname_00 (grep -Rni "def\ test_.*00(" tests/ | wc -l). At the same time, 44 tests hase testname_01 (grep -Rni "def\ test_.*01(" tests/ | wc -l).
  • Could we run isort for module files and tests?
  • Do we use any linter for module files and tests?