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 @@ def check_state(self) -> None:
         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 @@ def __init__(self) -> None:
         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 @@ def __init__(self) -> None:
     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,