util/ctx: extract suppress_oserror()

Extract the `suppress_oserror()` function from the ObjectManager and
make it available as utility for other code as well.

This also adds a bunch of tests that verify it works as expected.

test/mod/test_util_ctx.py

@@ -0,0 +1,43 @@
+#
+# Tests for the 'osbuild.util.ctx' module.
+#
+
+import errno
+import unittest
+
+from osbuild.util import ctx
+
+
+class TestUtilCtx(unittest.TestCase):
+    def test_suppress_oserror(self):
+        #
+        # Verify the `suppress_oserror()` function.
+        #
+
+        # Empty list and empty statement is a no-op.
+        with ctx.suppress_oserror():
+            pass
+
+        # Single errno matches raised errno.
+        with ctx.suppress_oserror(errno.EPERM):
+            raise OSError(errno.EPERM, "Operation not permitted")
+
+        # Many errnos match raised errno regardless of their order.
+        with ctx.suppress_oserror(errno.EPERM, errno.ENOENT, errno.ESRCH):
+            raise OSError(errno.EPERM, "Operation not permitted")
+        with ctx.suppress_oserror(errno.ENOENT, errno.EPERM, errno.ESRCH):
+            raise OSError(errno.EPERM, "Operation not permitted")
+        with ctx.suppress_oserror(errno.ENOENT, errno.ESRCH, errno.EPERM):
+            raise OSError(errno.EPERM, "Operation not permitted")
+
+        # Empty list re-raises exceptions.
+        with self.assertRaises(OSError):
+            with ctx.suppress_oserror():
+                raise OSError(errno.EPERM, "Operation not permitted")
+
+        # Non-matching lists re-raise exceptions.
+        with self.assertRaises(OSError):
+            with ctx.suppress_oserror(errno.ENOENT):
+                raise OSError(errno.EPERM, "Operation not permitted")
+            with ctx.suppress_oserror(errno.ENOENT, errno.ESRCH):
+                raise OSError(errno.EPERM, "Operation not permitted")