test: integrate pylint into the test infrastructure

Introduce a third test-group called `src` alongside `mod` and `run.
This will contain tests that run against the source code of osbuild.

This initial commit introduces `test/src/test_pylint.py` which will run
the python linter against all our sources.

test/src/test_pylint.py

@@ -0,0 +1,30 @@
+#
+# Run `pylint` on all python sources.
+#
+
+import subprocess
+import unittest
+
+from .. import test
+
+
+@unittest.skipUnless(test.TestBase.have_test_checkout(), "no test-checkout access")
+class TestPylint(test.TestBase, unittest.TestCase):
+    def test_pylint(self):
+        #
+        # Run `pylint` on all python sources. We simply use `find` to locate
+        # all `*.py` files, and then manually select the reverse-domain named
+        # modules we have.
+        #
+
+        path = self.locate_test_checkout()
+        options = "--errors-only"
+
+        subprocess.run(f"find {path} -type f -name '*.py' | xargs pylint {options}",
+                       shell=True, check=True)
+        subprocess.run(f"pylint {options}"
+                       + f" {path}/assemblers/*"
+                       + f" {path}/runners/*"
+                       + f" {path}/sources/*"
+                       + f" {path}/stages/*",
+                       shell=True, check=True)