Adds positional args for nox tests session

CONTRIBUTING.md

@@ -162,13 +162,21 @@ If you have nox installed, to run unit tests, type
 nox -s unit
 ```
 
-Alternatively, to run tests standalone with pytest, run,
+For individual tests, use:
+
+```bash
+nox -s tests -- tests/unit/test_costs.py::TestCosts::test_costs
+```
+
+which will run the specified cost, alternatively you can run all tests within a file by removing the trailing `::test_costs` in the above command.
+
+Alternatively, to run tests standalone with pytest, use:
 
 ```bash
 pytest --unit -v
 ```
 
-To run individual test files, you can use
+To run individual test files with nox, you can use
 
 ```bash
 pytest tests/unit/path/to/test --unit -v

noxfile.py

@@ -97,12 +97,20 @@ def notebooks_overwrite(session):
 
 @nox.session(name="tests")
 def run_tests(session):
-    """Run all the tests."""
+    """Run all or a user-defined set of tests."""
     session.install("-e", ".[all,dev]", silent=False)
     if PYBOP_SCHEDULED:
         session.run("pip", "install", f"pybamm=={PYBAMM_VERSION}", silent=False)
+    specific_tests = session.posargs if session.posargs else []
     session.run(
-        "pytest", "--unit", "--integration", "--nbmake", "--examples", "-n", "auto"
+        "pytest",
+        "--unit",
+        "--integration",
+        "--nbmake",
+        "--examples",
+        "-n",
+        "auto",
+        *specific_tests,
     )