Make php-saml compatible with phpunit >= 7.5.18

[pitbulk]

phpunit >= 7.5.18

This sebastianbergmann/phpunit#3968 introduces an incompatibility with php-saml test.

The redirectTo method has an exit which executed in a separated process now raises a

Test was run in child process and ended unexpectedly

I forced latest version of php-saml 3.X to use phpunit < 7.5.18
but we need to fix that in the future. The solutions:

• Add stay=true to the redirectTo , login and logout methods.
• Check on redirectTo method if a PHPUNIT_TEST constant is defined and if so, does not execute the exit and instead raise a controlled exception.