Update testing.rake with more accurate description of test task.

According to #5ecd12 change.

[ci skip]
Merge pull request #12688 from simi/patch-1

Update testing.rake with more accurate description of test task.
Clarify that visiting http://localhost:3000 should be done in a browser.
Merge pull request #12690 from durrantm/minor_README_change

Clarify that visiting http://localhost:3000 should be done in a browser. [ci skip]
Convert CDATA input to string before gsub'ing

Rails 3.2 API allowed arbitrary input for cdata_section;
this change re-introduces the old behaviour.
Merge pull request #12693 from carpodaster/convert-cdata-input-to-string

Convert CDATA input to string before gsub'ing
Update pull request instructions as per the new Github UI