dfilter-test.py: document parallelization support

Tests are independent and can be run in parallel using pytest-xdist
(https://github.com/pytest-dev/pytest-xdist), document it.

While at it, allow running the tests from other directories.

Change-Id: I3e55c549669f7d59d35cd64eca53680cea6dec2d
Reviewed-on: https://code.wireshark.org/review/26943
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

.gitignore

@@ -261,3 +261,4 @@ packaging/portableapps/appinfo.ini
 packaging/rpm/SPECS/wireshark.spec
 vgcore.*
 Wireshark.*
+.pytest_cache/

doc/README.display_filter

@@ -599,7 +599,14 @@ $ ./tools/dfilter-test.py testTVB
 # Run the the "test_contains_1" test from testTVB
 $ ./tools/dfilter-test.py testTVB.test_contains_1
 
-Note that dfilter-test.py should be run from the top of the
-Wireshark distribution, so it knows where to find the default
-tshark executable.
+# For faster, parallel tests, install the "pytest-xdist" first
+# (for example, using "pip install pytest-xdist"), then:
+$ pytest -nauto tools/dfilter-test.py
+
+# Run all tests in "testTVB", in parallel:
+$ pytest -nauto tools/dfilter-test.py -k testTVB
+
+By default dfilter-test.py assumes that the "tshark" binary is located in the
+current working directory. If that is not the case, set the environment variable
+WS_BIN_PATH to correct directory.
 

tools/dftestlib/dftest.py

@@ -28,8 +28,8 @@ class DFTest(unittest.TestCase):
         # names the trace file to use for the tests. It *should*
         # reside in dftestfiles
         assert not os.path.isabs(cls.trace_file)
-        cls.trace_file = os.path.join(os.getenv("SOURCE_DIR", "."), "tools",
-                "dftestfiles", cls.trace_file)
+        toolsdir = os.path.dirname(os.path.dirname(__file__))
+        cls.trace_file = os.path.join(toolsdir, "dftestfiles", cls.trace_file)
 
     @classmethod
     def tearDownClass(cls):