Add test for schema errors with tags named "root"

References to "root" should not internally be understood as a reference
to the root tag. The behavior of schemaErrors() was already correct, 
but seeing that this error came up in other areas of the code (e.g. the
Typescript generation), adding this test ensures that schemaErrors() 
stays correct.

test/schema/schemaErrorsTest.ts

@@ -266,6 +266,34 @@ describe("schemaErrors()", () => {
 				assert.strictEqual("prop", error.propName);
 				assert.strictEqual("unknown", error.referencedTag);
 			});
+
+			it("is returned when an unknown 'root' tag is referenced in a body prop", () => {
+				const schema: SchemaDefinition = {
+					root: root(prop("body", zeroOrMore("test"))),
+					blocks: {
+						["test"]: {
+							rawBody: false,
+							props: {
+								body: {
+									["prop"]: oneOrMore("root")
+								}
+							}
+						}
+					},
+					inline: {
+						["root"]: inline()
+					}
+				};
+
+				const errors = schemaErrors(schema);
+				assert.lengthOf(errors, 1);
+
+				const error = errors[0] as UndefinedBlockTagError;
+				assert.instanceOf(error, UndefinedBlockTagError);
+				assert.strictEqual("test", error.schemaTag);
+				assert.strictEqual("prop", error.propName);
+				assert.strictEqual("root", error.referencedTag);
+			});
 		});
 	});
 });