[FIX] tests: overflow on test page body

inherited from main CSS, would prevent from scrolling body and thus from
seeing all the test cases and test case contents (e.g. error messages
and stack traces)

addons/web/views/webclient_templates.xml

@@ -332,6 +332,9 @@
                     <t t-call="web.assets_common"/>
                     <t t-call="web.assets_backend"/>
 
+                    <style>
+                        body { overflow: auto !important; }
+                    </style>
                     <script type="text/javascript" id="qunit_config">
                         localStorage.clear();
                         QUnit.config.testTimeout = 5 * 60 * 1000;