Due to difficulties in testing whether pages are cached we split this bug off from bug 864098

The starts of a test are here but we think that the pageShow event is returning the wrong information in it's "persistent" property. Maybe we are best to use a .sjs file as we can force caching and read a unique value to check caching.

It is simple to use Etags etc to cache the page but detecting whether the page is cached is a completely different kettle of fish. This is made even more difficult because the cache toggling works for the current tab so any cache checks must be in the context of the current tab.

Using a combination of Etags and content checking works. This means that using a ^headers^ file doesn't work in order to make the browser cache test resource files. Try: https://tbpl.mozilla.org/?tree=Try&rev=283021e0dd56

Folding into patch for bug 864098