runtests.py: clean up PYTEST_ARGS (encode#6040)

1. `tests` and `--tb=short` is not necessary, since it is in
`pytest.addopts` already.
2. removes `-s` (shortcut for --capture=no): it is typically a good idea
   to not display output from successful tests.

runtests.py

@@ -7,8 +7,8 @@
 import pytest
 
 PYTEST_ARGS = {
-    'default': ['tests', '--tb=short', '-s', '-rw'],
-    'fast': ['tests', '--tb=short', '-q', '-s', '-rw'],
+    'default': [],
+    'fast': ['-q'],
 }
 
 FLAKE8_ARGS = ['rest_framework', 'tests']