ci.sh: switch to -r a to fix 3.8-dev error

Using '-ra' with Python 3.8-dev fails on `pytest: error: unrecognized
arguments: -ra`. This seems related to
https://bugs.python.org/issue26967.

Author: pquentin

ci.sh

@@ -120,7 +120,7 @@ else
 
     INSTALLDIR=$(python -c "import os, trio; print(os.path.dirname(trio.__file__))")
     cp ../setup.cfg $INSTALLDIR
-    pytest -W error -ra --junitxml=../test-results.xml --run-slow ${INSTALLDIR} --cov="$INSTALLDIR" --cov-config=../.coveragerc --verbose
+    pytest -W error -r a --junitxml=../test-results.xml --run-slow ${INSTALLDIR} --cov="$INSTALLDIR" --cov-config=../.coveragerc --verbose
 
     # Disable coverage on 3.8 until we run 3.8 on Windows CI too
     #   https://github.com/python-trio/trio/pull/784#issuecomment-446438407