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/utils.py

@@ -0,0 +1,64 @@
+import os
+import unittest
+
+from distutils.util import strtobool
+
+from transformers.file_utils import _tf_available, _torch_available
+
+
+try:
+    run_slow = os.environ["RUN_SLOW"]
+except KeyError:
+    # RUN_SLOW isn't set, default to skipping slow tests.
+    _run_slow_tests = False
+else:
+    # RUN_SLOW is set, convert it to True or False.
+    try:
+        _run_slow_tests = strtobool(run_slow)
+    except ValueError:
+        # More values are supported, but let's keep the message simple.
+        raise ValueError("If set, RUN_SLOW must be yes or no.")
+
+
+def slow(test_case):
+    """
+    Decorator marking a test as slow.
+
+    Slow tests are skipped by default. Set the RUN_SLOW environment variable
+    to a truthy value to run them.
+
+    """
+    if not _run_slow_tests:
+        test_case = unittest.skip("test is slow")(test_case)
+    return test_case
+
+
+def require_torch(test_case):
+    """
+    Decorator marking a test that requires PyTorch.
+
+    These tests are skipped when PyTorch isn't installed.
+
+    """
+    if not _torch_available:
+        test_case = unittest.skip("test requires PyTorch")(test_case)
+    return test_case
+
+
+def require_tf(test_case):
+    """
+    Decorator marking a test that requires TensorFlow.
+
+    These tests are skipped when TensorFlow isn't installed.
+
+    """
+    if not _tf_available:
+        test_case = unittest.skip("test requires TensorFlow")(test_case)
+    return test_case
+
+
+if _torch_available:
+    # Set the USE_CUDA environment variable to select a GPU.
+    torch_device = "cuda" if os.environ.get("USE_CUDA") else "cpu"
+else:
+    torch_device = None