Remove dependency on pytest for running tests (#2055)

* Switch to plain unittest for skipping slow tests.

Add a RUN_SLOW environment variable for running them.

* Switch to plain unittest for PyTorch dependency.

* Switch to plain unittest for TensorFlow dependency.

* Avoid leaking open files in the test suite.

This prevents spurious warnings when running tests.

* Fix unicode warning on Python 2 when running tests.

The warning was:

    UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal

* Support running PyTorch tests on a GPU.

Reverts 27e015bd.

* Tests no longer require pytest.

* Make tests pass on cuda

transformers/tests/tokenization_tests_commons.py

@@ -102,9 +102,11 @@ class CommonTestCases:
             with TemporaryDirectory() as tmpdirname:
 
                 filename = os.path.join(tmpdirname, u"tokenizer.bin")
-                pickle.dump(tokenizer, open(filename, "wb"))
+                with open(filename, "wb") as handle:
+                    pickle.dump(tokenizer, handle)
 
-                tokenizer_new = pickle.load(open(filename, "rb"))
+                with open(filename, "rb") as handle:
+                    tokenizer_new = pickle.load(handle)
 
             subwords_loaded = tokenizer_new.tokenize(text)