tests: Properly skip tests for isolation providers

The platform where we run our tests directly affects the isolation
providers we can choose. For instance, we cannot run Qubes tests on a
Windows/macOS platform, nor can we spawn containers in a Qubes platform,
if the `QUBES_CONVERSION` envvar has been specified.

This platform incompatibility was never an issue before, because
Dangerzone is capable of selecting the proper isolation provider under
the hood. However, with the addition of tests that target specific
isolation providers, it's possible that we may run by mistake a test
that does not apply to our platform.

To counter this, we employed `pytest.skipif()` guards around classes,
but we may omit those by mistake. Case in point, the `TestContainer`
class does not have such a guard, which means that we attempt to run
this test case on Qubes and it fails.

Add module-level guards in our isolation provider tests using pytest's
`pytest.skip("...", allow_module_level=True)` function, so that we make
such restrictions more explicit, and less easy to forget when we add a
new class.

tests/isolation_provider/base.py

@@ -16,7 +16,6 @@ TIMEOUT_STARTUP = 60  # Timeout in seconds until the conversion sandbox starts.
     os.environ.get("DUMMY_CONVERSION", False),
     reason="dummy conversions not supported",
 )
-@pytest.mark.skipif(not running_on_qubes(), reason="Not on a Qubes system")
 class IsolationProviderTest:
     def test_max_pages_server_enforcement(
         self,

tests/isolation_provider/test_container.py

@@ -9,6 +9,12 @@ from dangerzone.isolation_provider.qubes import is_qubes_native_conversion
 
 from .base import IsolationProviderTermination, IsolationProviderTest
 
+# Run the tests in this module only if we can spawn containers.
+if is_qubes_native_conversion():
+    pytest.skip("Qubes native conversion is enabled", allow_module_level=True)
+elif os.environ.get("DUMMY_CONVERSION", False):
+    pytest.skip("Dummy conversion is enabled", allow_module_level=True)
+
 
 @pytest.fixture
 def provider() -> Container:
@@ -44,12 +50,5 @@ class TestContainer(IsolationProviderTest):
     pass
 
 
-@pytest.mark.skipif(
-    os.environ.get("DUMMY_CONVERSION", False),
-    reason="cannot run for dummy conversions",
-)
-@pytest.mark.skipif(
-    is_qubes_native_conversion(), reason="Qubes native conversion is enabled"
-)
 class TestContainerTermination(IsolationProviderTermination):
     pass

tests/isolation_provider/test_dummy.py

@@ -11,6 +11,10 @@ from dangerzone.isolation_provider.dummy import Dummy
 
 from .base import IsolationProviderTermination
 
+# Run the tests in this module only if dummy conversion is enabled.
+if not os.environ.get("DUMMY_CONVERSION", False):
+    pytest.skip("Dummy conversion is not enabled", allow_module_level=True)
+
 
 class DummyWait(Dummy):
     """Dummy isolation provider that spawns a blocking process."""
@@ -34,10 +38,6 @@ def provider_wait() -> DummyWait:
     return DummyWait()
 
 
-@pytest.mark.skipif(
-    os.environ.get("DUMMY_CONVERSION", False) is False,
-    reason="can only run for dummy conversions",
-)
 class TestDummyTermination(IsolationProviderTermination):
     def test_failed(
         self,

tests/isolation_provider/test_qubes.py

@@ -9,14 +9,16 @@ from pytest_mock import MockerFixture
 
 from dangerzone.conversion import errors
 from dangerzone.document import Document
-from dangerzone.isolation_provider.qubes import (
-    Qubes,
-    is_qubes_native_conversion,
-    running_on_qubes,
-)
+from dangerzone.isolation_provider.qubes import Qubes, is_qubes_native_conversion
 
 from .base import IsolationProviderTermination, IsolationProviderTest
 
+# Run the tests in this module only if we can spawn disposable qubes.
+if not is_qubes_native_conversion():
+    pytest.skip("Qubes native conversion is not enabled", allow_module_level=True)
+elif os.environ.get("DUMMY_CONVERSION", False):
+    pytest.skip("Dummy conversion is enabled", allow_module_level=True)
+
 
 @pytest.fixture
 def provider() -> Qubes:
@@ -55,7 +57,6 @@ def provider_wait() -> QubesWait:
     return QubesWait()
 
 
-@pytest.mark.skipif(not running_on_qubes(), reason="Not on a Qubes system")
 class TestQubes(IsolationProviderTest):
     def test_out_of_ram(
         self,
@@ -82,12 +83,5 @@ class TestQubes(IsolationProviderTest):
             assert provider.get_proc_exception(proc) == errors.QubesQrexecFailed
 
 
-@pytest.mark.skipif(
-    os.environ.get("DUMMY_CONVERSION", False),
-    reason="cannot run for dummy conversions",
-)
-@pytest.mark.skipif(
-    not is_qubes_native_conversion(), reason="Qubes native conversion is not enabled"
-)
 class TestQubesTermination(IsolationProviderTermination):
     pass