Fix verbosity issue in DataDrivenTestCase

[elazarg]

Fix issue caused by #5459: .file was initialized lazily thus reported as empty. The fix is the assignment to self.file in the constructor. The field path is redundant.

There is no progress indication in the output:

mypy/test/testargs.py::ArgSuite::test_coherence PASSED
mypy/test/testargs.py::ArgSuite::test_executable_inference PASSED
mypy/test/testcheck.py::TypeCheckSuite::testEmptyFile <- /Users/elazarg/workspace/mypy/test-data/unit/check-basic.test PASSED
mypy/test/testcheck.py::TypeCheckSuite::testAssignmentAndVarDef <- /Users/elazarg/workspace/mypy/test-data/unit/check-basic.test PASSED

Additionally

• Renamed path consistently to file. I would prefer the former, but .file is used by unit tests.
• Since parse_test_case is effectively a setup method, it is more consistent not to pretend to pass it any argument other than case=self.

@msullivan @gvanrossum

[gvanrossum]

Thanks for the quick fix!