test: enable parallelism by default for pytest

Consistent with tools like "ninja", enable parallelism by default. It
can be disabled again with `pytest -n0`.

This makes pytest-xdist a dependency when pytest is used as test runner.
This does not affect the unittest-based test runnet (test/test.py).

Change-Id: I4dc905f097c57a869aa39487b15361502b431616
Reviewed-on: https://code.wireshark.org/review/31371
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

.travis.yml

@@ -72,6 +72,6 @@ before_script:
 script:
   - ninja
   - ninja test-programs
-  - pytest -nauto -v
+  - pytest -v
 after_script:
   - if [ -f run/tshark ]; then run/tshark --version; fi

appveyor.yml

@@ -58,7 +58,7 @@ before_test:
   - msbuild /m test-programs.vcxproj
 
 test_script:
-  - pytest -nauto
+  - pytest
 
 on_finish:
   - ps: |

pytest.ini

@@ -2,4 +2,4 @@
 minversion = 3.0
 testpaths = test
 python_files = suite_*.py group_*.py
-addopts = -ra
+addopts = -ra -nauto