Removing --release compat test.

- Leaving the one that checks that sampling follows the expected
distribution.
- Marking the python Unigram.train(..) test as slow
- The python Unigram.train(..) test now uses `big.txt` file.

bindings/python/conftest.py

@@ -0,0 +1,19 @@
+import pytest
+
+
+def pytest_addoption(parser):
+    parser.addoption("--runslow", action="store_true", default=False, help="run slow tests")
+
+
+def pytest_configure(config):
+    config.addinivalue_line("markers", "slow: mark test as slow to run")
+
+
+def pytest_collection_modifyitems(config, items):
+    if config.getoption("--runslow"):
+        # --runslow given in cli: do not skip slow tests
+        return
+    skip_slow = pytest.mark.skip(reason="need --runslow option to run")
+    for item in items:
+        if "slow" in item.keywords:
+            item.add_marker(skip_slow)