Added test to validate schema structure

refs https://github.com/TryGhost/Toolbox/issues/441

- this is only v1 of the test I would like but it validates the keys on
  a column definition are part of an allowlist
- this has already uncovered a bug with `maxLength` (vs `maxlength`)

ghost/core/test/unit/server/data/schema/schema.test.js

@@ -3,7 +3,77 @@ const _ = require('lodash');
 
 const schema = require('../../../../../core/server/data/schema/schema');
 
+const VALID_KEYS = {
+    bigInteger: [
+        'nullable'
+    ],
+    bool: [
+        'nullable',
+        'defaultTo'
+    ],
+    boolean: [
+        'nullable',
+        'defaultTo'
+    ],
+    dateTime: [
+        'nullable',
+        'index'
+    ],
+    integer: [
+        'nullable',
+        'unsigned',
+        'defaultTo',
+        'index'
+    ],
+    string: [
+        'maxlength',
+        'maxLength', // this is the result of a bug and should NOT be used, will be deleted soon
+        'nullable',
+        'primary',
+        'unique',
+        'validations',
+        'defaultTo',
+        'references',
+        'cascadeDelete',
+        'setNullDelete',
+        'index'
+    ],
+    text: [
+        'fieldtype',
+        'maxlength',
+        'nullable',
+        'validations'
+    ]
+};
+
 describe('schema validations', function () {
+    it('matches the required format', function () {
+        // The top-level export should be an object of table names to definitions
+        should(schema).be.Object();
+
+        // Each table should be an object, and each column should be an object
+        _.each(schema, function (table, tableName) {
+            should(table).be.Object();
+
+            _.each(table, function (column, columnName) {
+                if (['@@INDEXES@@', '@@UNIQUE_CONSTRAINTS@@'].includes(columnName)) {
+                    return;
+                }
+
+                // Ensure the column is an object
+                should(column).be.Object();
+
+                // Ensure the `type` key exists on a column
+                should.exist(column.type, `${tableName}.${columnName}.type should exist`);
+
+                // Ensure the column type is one of the ones we allow
+                should(column.type).be.equalOneOf(Object.keys(VALID_KEYS));
+
+                should(_.difference(Object.keys(column), [...VALID_KEYS[column.type], 'type'])).be.Array().with.length(0);
+            });
+        });
+    });
+
     it('has correct isIn validation structure', async function () {
         const tablesOnlyValidation = _.cloneDeep(schema);