cleanup: Prefer template strings

Template strings are much nicer than string concatenation, so use
them where possible; this excludes translatable strings and any
strings containing '/' (until we can depend on gettext >= 0.20[0]).

[0] https://savannah.gnu.org/bugs/?50920

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/612

js/perf/hwtest.js

@@ -273,7 +273,7 @@ function script_collectTimings(time) {
         else
             median = Math.round((times[len / 2 - 1] + times[len / 2]) / 2);
 
-        METRICS[timing + 'RedrawTime'].value = median;
+        METRICS[`${timing}RedrawTime`].value = median;
     }
 }