MDL-51881 behat: Set navbar to be absolutely positioned

All behat drivers are now replicating human behavior
so when the browser scrolls something into view it doesn't account
for fixed positioned elements that end up obscuring the item thus
leading to errors that could be avoided by scrolling an additional amount.

theme/bootstrapbase/less/moodle/debug.less

@@ -1,3 +1,12 @@
+// Here we are forcing the navbar to be absolutely positioned for behat, because
+// when driver scrolls something into view in browser it doesn't account
+// for fixed positioned elements that end up obscuring the item thus
+// leading to errors that could be avoided by scrolling an additional amount.
+body.behat-site {
+    .navbar-fixed-top {
+        position: absolute;
+    }
+}
 .phpinfo table,
 .phpinfo th,
 .phpinfo h2 {
@@ -89,4 +98,4 @@
 #page-footer .purgecaches,
 #page-footer .performanceinfo {
     text-align: center;
-}
+}