ipa-run-tests: replace chdir with plugin

The ipa-run-tests command used os.chdir() to change into the ipatests/
directory. The approach works for simple cases but breaks some pytest
features. For example it makes it impossible to selects tests by their
fully qualified test name.

Further more, coverage statistics break because path and module names
get messed up by chdir.

A name plugin takes care of adjusting paths relative to ipatests and to
add ipatests as base. It's now possible to run tests with qualified test
names, e.g.

  ipa-run-tests ipatests/test_ipalib/test_base.py::test_ReadOnly::test_lock

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Felipe Volpone <fbarreto@redhat.com>

ipatests/ipa-run-tests

@@ -40,10 +40,41 @@ os.environ['IPATEST_XUNIT_PATH'] = os.path.join(os.getcwd(), 'nosetests.xml')
 
 HERE = os.path.dirname(os.path.abspath(ipatests.__file__))
 
-args = sys.argv[1:]
-if not any('--confcutdir' in arg for arg in args):
-    args.insert(0, '--confcutdir={}'.format(HERE))
 
-os.chdir(HERE)
+class ArgsManglePlugin(object):
+    """Modify pytest arguments
 
-sys.exit(pytest.main(args))
+    * Add confcutdir if hasn't been set already
+    * Mangle paths to support tests both relative to basedir and ipatests/
+    * Default to ipatests/ if no tests are specified
+    """
+    def pytest_load_initial_conftests(self, early_config, parser, args):
+        # During initial loading, parser supports only basic options
+        ns = early_config.known_args_namespace
+        if not ns.confcutdir:
+            # add config cut directory to only load fixtures from ipatests/
+            args.insert(0, '--confcutdir={}'.format(HERE))
+
+        if not ns.file_or_dir:
+            # No file or directory found, run all tests
+            args.append(HERE)
+        else:
+            for name in ns.file_or_dir:
+                idx = args.index(name)
+                # split on pytest separator
+                # ipatests/test_ipaplatform/test_importhook.py::test_override
+                filename, sep, suffix = name.partition('::')
+                # a file or directory relative to cwd or already absolute
+                if os.path.exists(filename):
+                    continue
+                # a file or directory relative to ipatests package
+                args[idx] = sep.join((os.path.join(HERE, filename), suffix))
+
+        # rebuild early_config's known args with new args. The known args
+        # are used for initial conftest.py from ipatests, which adds
+        # additional arguments.
+        early_config.known_args_namespace = parser.parse_known_args(
+            args, namespace=early_config.option.copy())
+
+
+sys.exit(pytest.main(plugins=[ArgsManglePlugin()]))