Add prelim support for test-level markers (#5517)

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If you've fixed a bug or added code that should be tested, add a test to
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      test directory!

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If you do make documentation changes, make sure that the docs build and
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------------------------------------------------------------------------------------------------------------

**Context:** This PR introduces new test-level markers for PennyLane,
which can begin use for indication of where interactions take-place in
the given suite. This forms part of the work on improving testing across
the packages
[sc-61416]

**Description of the Change:** Add 3 new test-level markers for pytest:
`unit_test`, `integration_test` and `system_test`. Some sample tests
that fit into the 3 categories are added for examination purposes.

**Benefits:** This will help us to structure our test-suite to better
reflect localised tests from intermodule and interpackage tests.

**Possible Drawbacks:** 3 new markers to work with.

**Related GitHub Issues:**

---------

Co-authored-by: Thomas R. Bromley <49409390+trbromley@users.noreply.github.com>

doc/releases/changelog-dev.md

@@ -9,6 +9,9 @@
 
 <h3>Improvements 🛠</h3>
 
+* Add support for 3 new pytest markers: `unit`, `integration` and `system`.
+  [(#5517)](https://github.com/PennyLaneAI/pennylane/pull/5517)
+
 * The sorting order of parameter-shift terms is now guaranteed to resolve ties in the absolute value with the sign of the shifts.
   [(#5582)](https://github.com/PennyLaneAI/pennylane/pull/5582)
 
@@ -175,5 +178,6 @@ Soran Jahangiri,
 Korbinian Kottmann,
 Christina Lee,
 Vincent Michaud-Rioux,
+Lee James O'Riordan,
 Kenya Sakka,
 David Wierichs.

tests/numpy/test_numpy_random.py

@@ -43,6 +43,7 @@
 general_gen = random.default_rng()
 
 
+@pytest.mark.unit
 class TestGeneratorDistributions:
     @pytest.mark.parametrize("distribution", distributions_no_extra_input)
     def test_generator_distributions(self, distribution):
@@ -61,6 +62,7 @@ def test_generator_distributions(self, distribution):
         assert output.requires_grad is False
 
 
+@pytest.mark.unit
 class Test_default_rng:
     def test_no_input(self):
         """Tests that np.random.default_rng() returns a generator object when

tests/numpy/test_numpy_wrapper.py

@@ -25,6 +25,7 @@
 from pennylane.numpy.tensor import tensor_to_arraybox
 
 
+@pytest.mark.unit
 class TestExtractTensors:
     """Tests for the extract_tensors function"""
 
@@ -56,6 +57,7 @@ def test_iterable_with_unpatched_numpy_arrays(self):
         assert res[1] is arr2
 
 
+@pytest.mark.unit
 class TestTensor:
     """Tests for the Tensor(ndarray) subclass"""
 
@@ -141,6 +143,7 @@ def test_numpy_to_arraybox(self):
 ]
 
 
+@pytest.mark.unit
 class TestNumpyIntegration:
     """Test that the wrapped NumPy functionality integrates well
     with standard NumPy functions."""
@@ -428,6 +431,7 @@ def __call__(self, *args, **kwargs):
         assert len(res) == 2
 
 
+@pytest.mark.integration
 class TestAutogradIntegration:
     """Test autograd works with the new tensor subclass"""
 
@@ -458,6 +462,7 @@ def cost(x):
             grad_fn(arr1)
 
 
+@pytest.mark.unit
 class TestScalarHashing:
     """Test for the hashing capability of scalar arrays."""
 
@@ -505,13 +510,15 @@ def test_nonzero_dim_arrays_non_hashable(self):
 class TestNumpyConversion:
     """Tests for the tensor.unwrap() and tensor.numpy() methods"""
 
+    @pytest.mark.unit
     def test_convert_scalar_array(self):
         """Test that a scalar array converts to a python literal"""
         data = np.array(1.543)
         res = data.unwrap()
         assert res == data.item()
         assert isinstance(res, float)
 
+    @pytest.mark.unit
     def test_convert_array(self):
         """Test that a numpy array successfully converts"""
         data = np.array([1, 2, 3])
@@ -522,6 +529,7 @@ def test_convert_array(self):
         assert isinstance(res, np.ndarray)
         assert not isinstance(res, np.tensor)
 
+    @pytest.mark.system
     def test_single_gate_parameter(self):
         """Test that when supplied a PennyLane tensor, a QNode passes an
         unwrapped tensor as the argument to a gate taking a single parameter"""
@@ -545,6 +553,7 @@ def circuit(phi=None):
             assert op.name == "RX"
             assert op.parameters == [p]
 
+    @pytest.mark.system
     def test_multiple_gate_parameter(self):
         """Test that when supplied a PennyLane tensor, a QNode passes arguments
         as unwrapped tensors to a gate taking multiple parameters"""

tests/pytest.ini

@@ -1,5 +1,8 @@
 [pytest]
 markers =
+    unit: marks tests as unit-level tests (select with '-m "unit"')
+    integration: marks tests as integration-level tests (select with '-m "integration"')
+    system: marks tests as system-level tests (select with '-m "system"')
     core: marks tests for core testing (select with '-m "core"')
     autograd: marks tests for autograd testing (select with '-m "autograd"')
     torch: marks tests for torch testing (select with '-m "core"')