Bug 1024715 - Create test_firefox_accounts_login to validate user authen...

...tication from Settings and FMD R=zacc,bebe
These 3 tests need to be split into separate files because the ManifestParser requires it.

The file names also need to be added into the manifest.ini file too so the tests are picked up by test runners and tags set.

Also, would you prefer to add these into their own folder, eg `functional/fx_accounts`  ?  I think it's OK to consider the fxaccounts app as its own I think.