Fixed failing tests because of circular dependency

packages/grafana-ui/src/themes/ThemeContext.tsx

 import React from 'react';
-import { getTheme } from './index';
+import { getTheme } from './getTheme';
 import { GrafanaThemeType, Themeable } from '../types/theme';
 
 type Omit<T, K> = Pick<T, Exclude<keyof T, K>>;

packages/grafana-ui/src/themes/getTheme.ts

+import darkTheme from './dark';
+import lightTheme from './light';
+import { GrafanaTheme } from '../types/theme';
+
+let themeMock: ((name?: string) => GrafanaTheme) | null;
+
+export const getTheme = (name?: string) =>
+  (themeMock && themeMock(name)) || (name === 'light' ? lightTheme : darkTheme);
+
+export const mockTheme = (mock: (name?: string) => GrafanaTheme) => {
+  themeMock = mock;
+  return () => {
+    themeMock = null;
+  };
+};

packages/grafana-ui/src/themes/index.ts

-import darkTheme from './dark';
-import lightTheme from './light';
-import { GrafanaTheme } from '../types/theme';
 import { ThemeContext, withTheme } from './ThemeContext';
+import { getTheme, mockTheme } from './getTheme';
 
-let themeMock: ((name?: string) => GrafanaTheme) | null;
-
-export let getTheme = (name?: string) => (themeMock && themeMock(name)) || (name === 'light' ? lightTheme : darkTheme);
-
-export const mockTheme = (mock: (name?: string) => GrafanaTheme) => {
-  themeMock = mock;
-  return () => {
-    themeMock = null;
-  };
-};
-
-export { ThemeContext, withTheme };
+export { ThemeContext, withTheme, mockTheme, getTheme };