tests: Use one store per class instead of per test method

For stages testing it is too slow to rebuild the a image containing
@core packages every time. Let's just reuse the a image for all tests.
This should speed up the test running time a LOT.

test/osbuildtest.py

@@ -13,21 +13,24 @@ class TestCase(unittest.TestCase):
     """A TestCase to test running the osbuild program.
 
     Each test case can use `self.run_osbuild()` to run osbuild. A temporary
-    store is used, which can be accessed through `self.store`.
+    store is used, which can be accessed through `self.store`. The store is
+    persistent for whole test class due to performance concerns.
 
     To speed up local development, OSBUILD_TEST_STORE can be set to an existing
     store. Note that this might make tests dependant of each other. Do not use
     it for actual testing.
     """
 
-    def setUp(self):
-        self.store = os.getenv("OSBUILD_TEST_STORE")
-        if not self.store:
-            self.store = tempfile.mkdtemp(dir="/var/tmp")
+    @classmethod
+    def setUpClass(cls):
+        cls.store = os.getenv("OSBUILD_TEST_STORE")
+        if not cls.store:
+            cls.store = tempfile.mkdtemp(dir="/var/tmp")
 
-    def tearDown(self):
+    @classmethod
+    def tearDownClass(cls):
         if not os.getenv("OSBUILD_TEST_STORE"):
-            shutil.rmtree(self.store)
+            shutil.rmtree(cls.store)
 
     def run_osbuild(self, pipeline, input=None):
         osbuild_cmd = ["python3", "-m", "osbuild", "--json", "--store", self.store, "--libdir", ".", pipeline]