chore: update mocks, move configs to jest dir

jest/__mocks__/fileMock.js

@@ -0,0 +1,3 @@
+'use strict';
+
+module.exports = 'file';

jest/__mocks__/gatsby.js

@@ -0,0 +1,29 @@
+'use strict';
+
+const React = require('react');
+
+const gatsby = jest.requireActual('gatsby');
+
+module.exports = {
+  ...gatsby,
+  graphql: jest.fn(),
+  Link: jest.fn().mockImplementation(
+    ({
+      activeClassName,
+      activeStyle,
+      getProps,
+      innerRef,
+      ref,
+      replace,
+      to,
+      ...rest
+    }) => (
+      React.createElement('a', {
+        ...rest,
+        href: to,
+      })
+    )
+  ),
+  StaticQuery: jest.fn(),
+  useStaticQuery: jest.fn(),
+};