Implemented vfsStream.
Temporarily use process isolation for Kernel test suite.
Fixed PHP Notice: Undefined index: $GLOBALS['base_url'].
[interim] Fix process isolation infinitely blocks on Windows.

https://github.com/sebastianbergmann/phpunit/pull/1340
[interim] Fix standard IO streams are not defined in process isolation.

https://github.com/sebastianbergmann/phpunit/pull/1348
Avoid namespace discovery for tests running in isolation.