Implement PcntlForkJobRunner

[staabm]

running
php phpunit tests/end-to-end/phar/tests/standard/GreeterTest.php -c tests/end-to-end/phar/phpunit.xml
with a fork Time: 00:00.005, Memory: 16.00 MB
with a subprocess Time: 00:00.031, Memory: 16.00 MB

Todo

• implement PcntlForkJobRunner
• make CI run with pcntl
• fix tests
• add attribute parameter to RunTestsInSeparateProcesses, RunInSeparateProcess to opt-in fork-mode

[staabm]

currently one test is failling:

1) /Users/staabm/workspace/phpunit/tests/end-to-end/regression/5592-process-isolation.phpt
Failed asserting that string matches format description.
--- Expected
+++ Actual
@@ @@
 
 --
 
-There was 1 risky test:
+There were 2 risky tests:
 
 1) PHPUnit\TestFixture\Issue5592Test::testRemovedErrorHandler
 Test code or tested code removed error handlers other than its own
@@ @@
 
 /Users/staabm/workspace/phpunit/tests/end-to-end/regression/5592/Issue5592Test.php:33
 
+2) PHPUnit\TestFixture\Issue5592Test::testRemovedExceptionHandler
+Test code or tested code removed exception handlers other than its own
+
+/Users/staabm/workspace/phpunit/tests/end-to-end/regression/5592/Issue5592Test.php:52
+
 FAILURES!
-Tests: 6, Assertions: 6, Failures: 4, Risky: 1.
+Tests: 6, Assertions: 6, Failures: 4, Risky: 2.

/Users/staabm/workspace/phpunit/tests/end-to-end/regression/5592-process-isolation.phpt:45
/Users/staabm/workspace/phpunit/src/Framework/TestSuite.php:374
/Users/staabm/workspace/phpunit/src/Framework/TestSuite.php:374
/Users/staabm/workspace/phpunit/src/TextUI/TestRunner.php:64
/Users/staabm/workspace/phpunit/src/TextUI/Application.php:228

FAILURES!
Tests: 4591, Assertions: 12200, Failures: 1, Skipped: 23, Incomplete: 1.

... I wonder why 0e4f6d8 started to ignore "Test code or tested code did not remove its own error handlers" when test run in isolation, but did not the same for "Test code or tested code removed error handlers other than its own" ?

edit: answered in 0e4f6d8#r163466240