Move container-specific method from base class

Move the `is_runtime_available()` method from the base
`IsolationProvider` class, and into the `Dummy` provider class. This
method was originally defined in the base class, in order to be mocked
in our tests for the `Dummy` provider. There's no reason for the `Qubes`
class to have it though, so we can just move it to the `Dummy` provider.

dangerzone/gui/main_window.py

@@ -500,6 +500,7 @@ class WaitingWidgetContainer(WaitingWidget):
         error: Optional[str] = None
 
         try:
+            assert isinstance(self.dangerzone.isolation_provider, (Dummy, Container))
             self.dangerzone.isolation_provider.is_runtime_available()
         except NoContainerTechException as e:
             log.error(str(e))

dangerzone/isolation_provider/base.py

@@ -93,10 +93,6 @@ class IsolationProvider(ABC):
         else:
             self.proc_stderr = subprocess.DEVNULL
 
-    @staticmethod
-    def is_runtime_available() -> bool:
-        return True
-
     @abstractmethod
     def install(self) -> bool:
         pass

dangerzone/isolation_provider/dummy.py

@@ -39,6 +39,10 @@ class Dummy(IsolationProvider):
     def install(self) -> bool:
         return True
 
+    @staticmethod
+    def is_runtime_available() -> bool:
+        return True
+
     def start_doc_to_pixels_proc(self, document: Document) -> subprocess.Popen:
         cmd = [
             sys.executable,

tests/gui/test_main_window.py

@@ -30,6 +30,7 @@ from dangerzone.isolation_provider.container import (
     NoContainerTechException,
     NotAvailableContainerTechException,
 )
+from dangerzone.isolation_provider.dummy import Dummy
 
 from .test_updater import assert_report_equal, default_updater_settings
 
@@ -510,9 +511,9 @@ def test_not_available_container_tech_exception(
 ) -> None:
     # Setup
     mock_app = mocker.MagicMock()
-    dummy = mocker.MagicMock()
+    dummy = Dummy()
-
+    fn = mocker.patch.object(dummy, "is_runtime_available")
-    dummy.is_runtime_available.side_effect = NotAvailableContainerTechException(
+    fn.side_effect = NotAvailableContainerTechException(
         "podman", "podman image ls logs"
     )