test: add new testutil.assert_jsonschema_error_contains() helper

This commit adds a new helper `assert_jsonschema_error_contains()`
to `testutil` and uses it everywhere where we check errors from
jsonschema.

test/mod/test_testutil_jsonschema.py

@@ -0,0 +1,51 @@
+import re
+
+import pytest
+
+import osbuild.meta
+from osbuild import testutil
+
+fake_schema = {
+    "type": "object",
+    "required": ["name"],
+}
+
+
+@pytest.fixture(name="validation_error")
+def validation_error_fixture():
+    schema = osbuild.meta.Schema(fake_schema, "fake-schema")
+    res = schema.validate({"not": "name"})
+    assert res.valid is False
+    return res
+
+
+def test_assert_jsonschema_error_contains(validation_error):
+    expected_err = "'name' is a required property"
+    testutil.assert_jsonschema_error_contains(validation_error, expected_err)
+
+
+def test_assert_jsonschema_error_regex(validation_error):
+    expected_err = re.compile("'.*' is a required property")
+    testutil.assert_jsonschema_error_contains(validation_error, expected_err)
+
+
+def test_assert_jsonschema_error_not_contains(validation_error):
+    with pytest.raises(AssertionError, match=r'not-in-errs not found in \['):
+        testutil.assert_jsonschema_error_contains(validation_error, "not-in-errs")
+
+
+def test_assert_jsonschema_error_not_found_re(validation_error):
+    expected_err_re = re.compile("not-in-errs")
+    with pytest.raises(AssertionError, match=r"re.*not found in"):
+        testutil.assert_jsonschema_error_contains(validation_error, expected_err_re)
+
+
+def test_assert_jsonschema_error_num_errs(validation_error):
+    expected_err = "'name' is a required property"
+    testutil.assert_jsonschema_error_contains(validation_error, expected_err, expected_num_errs=1)
+
+
+def test_assert_jsonschema_error_num_errs_wrong(validation_error):
+    expected_err = "'name' is a required property"
+    with pytest.raises(AssertionError, match=r'expected exactly 99 errors in'):
+        testutil.assert_jsonschema_error_contains(validation_error, expected_err, expected_num_errs=99)