Fix test cookie may be set multiple times #15225

• Make sure to not set a test cookie per site but instead use one test cookie overall
• Only set test cookie on IE or browsers that support modal dialog
• Delete the test cookie after it has been set https://github.com/matomo-org/matomo/issues/15161#issuecomment-560543852

fix https://github.com/matomo-org/matomo/issues/15209

_pk_test cookie..undefined is still be set

Two test cookies are still set in Firefox with Matomo 3.13.3.
I do not use the disable-cookies setting, but multiple domains.
These are the activated settings:
_paq.push (["setCookieDomain", " ..."]);
         _paq.push (["setDomains", [" ....", "* ...."]]);
         _paq.push (["enableCrossDomainLinking"]);
         _paq.push (['trackPageView']);
         _paq.push (['enableLinkTracking']);