Make unit tests in test_test.py independent of each other

[utsavm9]

Currently, these 5 tests in test_test.py want the file __cell_coverage_test.py to not exist when each of those tests are run. These tests when call another function to make _cell_coverage_test.py.

• test_basic_runWithReturn_pass
• test_basic_runWithReturn_fail
• test_basic_runWithReport_pass
• test_basic_runWithHTMLReturn_pass
• test_basic_runWithHTMLReturn_fail

As such, these tests have been placed in a class TestCoverage and run with --dist=loadscope when executing make testpy-distributed. This causes the tests to be run sequentially.

If we can make these tests independent of each other, then there would be no need to group them in a class and the --dist option can also be removed.

[ceball]

Note that the real fix might well be in the underlying function(s). E.g. maybe should be using a unique temporary file.

[ceball]

See also #125