tests: webui: Load qunit only once
    
    webui unit tests fail with grunt-contrib-qunit:
    ```
    Testing test/all_tests.html
    
    >> Error: Error: QUnit has already been defined.
    >>     at exportQUnit (file:///home/test/freeipa/install/ui/js/qunit.js:2475:12)
    >>     at file:///home/test/freeipa/install/ui/js/qunit.js:2946:3
    >>     at file:///home/test/freeipa/install/ui/js/qunit.js:5061:2
    
    >> Error: TypeError: Cannot set properties of undefined (setting 'reorder')
    >>     at <anonymous>:175:24
    >>     at runFactory (file:///home/test/freeipa/install/ui/js/dojo/dojo.js:1:17157)
    >>     at execModule (file:///home/test/freeipa/install/ui/js/dojo/dojo.js:1:19541)
    >>     at file:///home/test/freeipa/install/ui/js/dojo/dojo.js:1:20002
    >>     at guardCheckComplete (file:///home/test/freeipa/install/ui/js/dojo/dojo.js:1:19707)
    >>     at checkComplete (file:///home/test/freeipa/install/ui/js/dojo/dojo.js:1:19854)
    >>     at onLoadCallback (file:///home/test/freeipa/install/ui/js/dojo/dojo.js:1:22296)
    >>     at HTMLScriptElement.onLoad (file:///home/test/freeipa/install/ui/js/dojo/dojo.js:1:26209)
    ```
    
    Load `qunit` with `dojo.require` that among other useful things helps
    > Preventing loading Dojo packages twice.
      dojo.require will simply return if the package is already loaded.
    
    See also https://github.com/gruntjs/grunt-contrib-qunit#loading-qunit-with-amd
    
    Related: https://pagure.io/freeipa/issue/9329
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>