Locate test data from __file__

Find c_files and fake_libc_include from __file__ of the unit test,
rather than expecting the test suite to be run from a particular
directory.
diff --git a/tests/test_c_parser.py b/tests/test_c_parser.py
index bb85233..d695b47 100644
--- a/tests/test_c_parser.py
+++ b/tests/test_c_parser.py
@@ -1518,13 +1518,10 @@
         """ Find a c file by name, taking into account the current dir can be
             in a couple of typical places
         """
-        fullnames = [
-            os.path.join('c_files', name),
-            os.path.join('tests', 'c_files', name)]
-        for fullname in fullnames:
-            if os.path.exists(fullname):
-                return open(fullname, 'rU')
-        assert False, "Unreachable"
+        testdir = os.path.dirname(__file__)
+        name = os.path.join(testdir, 'c_files', name)
+        assert os.path.exists(name)
+        return open(name, 'rU')
 
     def test_whole_file(self):
         # See how pycparser handles a whole, real C file.