test/unit: extract common code

Create a new plugintest.PluginTest class that shares the plugin
loading code that is common to all plugin testing. Adapt the
existing (hub, builder) tests.
Also correct the name for test_builder.TestHubPlugin to
TestBuilderPlugin.

test/unit/plugintest.py

@@ -0,0 +1,46 @@
+#
+# Test Infrastructure
+#
+
+
+import imp
+import os
+import unittest
+
+
+class PluginTest(unittest.TestCase):
+    """Base class for Plugin tests
+
+    Use the PluginTest.load_plugin class decorator to automatically
+    load a plugin. If said decorator has not been specified, the
+    `plugin` property will be set to `None`.
+    """
+
+    @staticmethod
+    def load_plugin(plugin_type):
+        def decorator(klass):
+            setattr(klass, "_plugin_type", plugin_type)
+            return klass
+        return decorator
+
+    def _load_plugin(self):
+        plugin_type = getattr(self, "_plugin_type", None)
+        if not plugin_type:
+            return None
+
+        root = os.getenv("GITHUB_WORKSPACE", os.getcwd())
+        haystack = os.path.join(root, "plugins", plugin_type)
+        fp, path, desc = imp.find_module("osbuild", [haystack])
+
+        try:
+            return imp.load_module("osbuild", fp, path, desc)
+        finally:
+            fp.close()
+
+    def setUp(self):
+        """Setup plugin testing environment
+
+        Will load the specified plugin if the derived class has
+        been decorated with `Plugintest.load_plugin()`.
+        """
+        self.plugin = self._load_plugin()