Improve test level mechanism, apply to very slow svmbir tests

conftest.py

@@ -27,8 +27,25 @@ def pytest_addoption(parser, pluginmanager):
     Level definitions:
       1  Critical tests only
       2  Skip tests that do have a significant impact on coverage
-      3  All tests
+      3  All standard tests
+      4  Run all tests, including those marked as slow to run
     """
     parser.addoption(
         "--level", action="store", default=3, type=int, help="Set test level to be run"
     )
+
+
+def pytest_configure(config):
+    """Add marker description."""
+    config.addinivalue_line("markers", "slow: mark test as slow to run")
+
+
+def pytest_collection_modifyitems(config, items):
+    """Skip slow tests depending on selected testing level."""
+    if config.getoption("--level") >= 4:
+        # don't skip tests at level 4 or higher
+        return
+    level_skip = pytest.mark.skip(reason="test not appropriate for selected level")
+    for item in items:
+        if "slow" in item.keywords:
+            item.add_marker(level_skip)

scico/test/linop/xray/test_svmbir.py

@@ -88,7 +88,6 @@ def make_A(
     delta_channel=None,
     delta_pixel=None,
 ):
-
     angles = make_angles(num_angles)
     A = XRayTransform(
         im.shape,
@@ -154,6 +153,7 @@ def test_adjoint(
     adjoint_test(A)
 
 
+@pytest.mark.slow
 @pytest.mark.skipif(device.platform != "cpu", reason="test hangs on gpu")
 def test_prox(
     is_3d,
@@ -185,6 +185,7 @@ def test_prox(
     prox_test(v, f, f.prox, alpha=0.25, rtol=5e-4)
 
 
+@pytest.mark.slow
 @pytest.mark.skipif(device.platform != "cpu", reason="test hangs on gpu")
 def test_prox_weights(
     is_3d,