[FIX] web: restore mobile responsive viewport in mobile test suite

After commit odoo/enterprise@8a38a60b1f
mobile tests didn't run correctly because of missing viewport META in HEAD.

This commit add the missing META. This META must be before all
JavaScript file to ensure that JavaScript also know that we work in
a viewport.

Note that "runbot" run tests in a chromium headless with a size screen
reduced and so it worked without this fix.

closes odoo/odoo#41183

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/web/views/webclient_templates.xml

@@ -726,6 +726,7 @@
             <t t-set="html_data" t-value="{'style': 'height: 100%;'}"/>
             <t t-set="title">Web Mobile Tests</t>
             <t t-set="head">
+                <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no"/>
                 <t t-call="web.js_tests_assets"/>
                 <script type="text/javascript" src="/web/static/lib/jquery.touchSwipe/jquery.touchSwipe.js"></script>