Include performance info also when the user is redirected

[dmonllao]

Following https://tracker.moodle.org/browse/MDL-42447 where we will add a new var (CFG or const) to show the performance info also when redirections are executed.

• We have 303 redirections, which, by default, are followed by jmeter, getting the last page's performance info (we can change this behaviour disabling follow redirects)
• We have 200 redirections, where we show a message to the user and we add a meta with a refresh value

The current test plan is affected by the second issue, when we send the data to add a new post discussion the performance info is not shown as we end up with a 200 HTTP response and a message stating that the user have 30 minutes to edit the post.

We would be affected by the first one if, for example, we test the add an activity instance performance, after adding it we perform a redirect with a 303 HTTP response which leads to the course main page or the activity page, depending on whether the selected submit button.

This issue is about adding the defined('MDL_PERF_TEST') or whatever var we end up using to the config.php template.

[dmonllao]

https://github.com/dmonllao/moodle-performance-comparison/commit/0d41047b818d662b646dc9f7c0cd0db94a8c5743 can be backported to 2.5

Pending on MDL-42447 being accepted.

This will require, along with the latests changes a has upgrade once moodle 2.6 is released.

[dmonllao]

merged in 2.5 and master