jsonschema: add more schema fields

Add additional fields to Schema which are mentioned in the spec but
do not affect validation.

Change-Id: I89488261a00c207d01cd7fc59d782f43692fe528
Reviewed-on: https://go-review.googlesource.com/c/tools/+/674976
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>

jsonschema/testdata/draft2020-12/default.json

@@ -0,0 +1,82 @@
+[
+    {
+        "description": "invalid type for default",
+        "schema": {
+            "$schema": "https://json-schema.org/draft/2020-12/schema",
+            "properties": {
+                "foo": {
+                    "type": "integer",
+                    "default": []
+                }
+            }
+        },
+        "tests": [
+            {
+                "description": "valid when property is specified",
+                "data": {"foo": 13},
+                "valid": true
+            },
+            {
+                "description": "still valid when the invalid default is used",
+                "data": {},
+                "valid": true
+            }
+        ]
+    },
+    {
+        "description": "invalid string value for default",
+        "schema": {
+            "$schema": "https://json-schema.org/draft/2020-12/schema",
+            "properties": {
+                "bar": {
+                    "type": "string",
+                    "minLength": 4,
+                    "default": "bad"
+                }
+            }
+        },
+        "tests": [
+            {
+                "description": "valid when property is specified",
+                "data": {"bar": "good"},
+                "valid": true
+            },
+            {
+                "description": "still valid when the invalid default is used",
+                "data": {},
+                "valid": true
+            }
+        ]
+    },
+    {
+        "description": "the default keyword does not do anything if the property is missing",
+        "schema": {
+            "$schema": "https://json-schema.org/draft/2020-12/schema",
+            "type": "object",
+            "properties": {
+                "alpha": {
+                    "type": "number",
+                    "maximum": 3,
+                    "default": 5
+                }
+            }
+        },
+        "tests": [
+            {
+                "description": "an explicit property value is checked against maximum (passing)",
+                "data": { "alpha": 1 },
+                "valid": true
+            },
+            {
+                "description": "an explicit property value is checked against maximum (failing)",
+                "data": { "alpha": 5 },
+                "valid": false
+            },
+            {
+                "description": "missing properties are not filled in with the default",
+                "data": {},
+                "valid": true
+            }
+        ]
+    }
+]