Phabricator::generateNewTester() uses a example.com email address, but this fails when the phabricator instance has auth.email-domains configured. For me this caused unit test failures when running `arc diff` for D14318
Specific errors I saw:
```
FAIL ManiphestTaskTestCase::testTaskAdjacentBlocks
EXCEPTION (Exception): Email address must be @rajant.com
#0 /home/jsmith/code/phabricator-install/phabricator/src/applications/people/editor/PhabricatorUserEditor.php(60): PhabricatorUserEditor->willAddEmail(Object(PhabricatorUserEmail))
#1 /home/jsmith/code/phabricator-install/phabricator/src/infrastructure/testing/PhabricatorTestCase.php(197): PhabricatorUserEditor->createNewUser(Object(PhabricatorUser), Object(PhabricatorUserEmail))
#2 /home/jsmith/code/phabricator-install/phabricator/src/applications/maniphest/__tests__/ManiphestTaskTestCase.php(94): PhabricatorTestCase->generateNewTestUser()
#3 [internal function]: ManiphestTaskTestCase->testTaskAdjacentBlocks()
#4 /home/jsmith/code/arc/arcanist/src/unit/engine/phutil/PhutilTestCase.php(492): call_user_func_array(Array, Array)
#5 /home/jsmith/code/arc/arcanist/src/unit/engine/PhutilUnitTestEngine.php(69): PhutilTestCase->run()
#6 /home/jsmith/code/arc/arcanist/src/unit/engine/ArcanistConfigurationDrivenUnitTestEngine.php(143): PhutilUnitTestEngine->run()
#7 /home/jsmith/code/arc/arcanist/src/workflow/ArcanistUnitWorkflow.php(175): ArcanistConfigurationDrivenUnitTestEngine->run()
#8 /home/jsmith/code/arc/arcanist/src/workflow/ArcanistDiffWorkflow.php(1330): ArcanistUnitWorkflow->run()
#9 /home/jsmith/code/arc/arcanist/src/workflow/ArcanistDiffWorkflow.php(1218): ArcanistDiffWorkflow->runUnit()
#10 /home/jsmith/code/arc/arcanist/src/workflow/ArcanistDiffWorkflow.php(474): ArcanistDiffWorkflow->runLintUnit()
#11 /home/jsmith/code/arc/arcanist/scripts/arcanist.php(382): ArcanistDiffWorkflow->run()
#12 {main}
FAIL ManiphestTaskTestCase::testTaskReordering
EXCEPTION (Exception): Email address must be @rajant.com
#0 /home/jsmith/code/phabricator-install/phabricator/src/applications/people/editor/PhabricatorUserEditor.php(60): PhabricatorUserEditor->willAddEmail(Object(PhabricatorUserEmail))
#1 /home/jsmith/code/phabricator-install/phabricator/src/infrastructure/testing/PhabricatorTestCase.php(197): PhabricatorUserEditor->createNewUser(Object(PhabricatorUser), Object(PhabricatorUserEmail))
#2 /home/jsmith/code/phabricator-install/phabricator/src/applications/maniphest/__tests__/ManiphestTaskTestCase.php(12): PhabricatorTestCase->generateNewTestUser()
#3 [internal function]: ManiphestTaskTestCase->testTaskReordering()
#4 /home/jsmith/code/arc/arcanist/src/unit/engine/phutil/PhutilTestCase.php(492): call_user_func_array(Array, Array)
#5 /home/jsmith/code/arc/arcanist/src/unit/engine/PhutilUnitTestEngine.php(69): PhutilTestCase->run()
#6 /home/jsmith/code/arc/arcanist/src/unit/engine/ArcanistConfigurationDrivenUnitTestEngine.php(143): PhutilUnitTestEngine->run()
#7 /home/jsmith/code/arc/arcanist/src/workflow/ArcanistUnitWorkflow.php(175): ArcanistConfigurationDrivenUnitTestEngine->run()
#8 /home/jsmith/code/arc/arcanist/src/workflow/ArcanistDiffWorkflow.php(1330): ArcanistUnitWorkflow->run()
#9 /home/jsmith/code/arc/arcanist/src/workflow/ArcanistDiffWorkflow.php(1218): ArcanistDiffWorkflow->runUnit()
#10 /home/jsmith/code/arc/arcanist/src/workflow/ArcanistDiffWorkflow.php(474): ArcanistDiffWorkflow->runLintUnit()
#11 /home/jsmith/code/arc/arcanist/scripts/arcanist.php(382): ArcanistDiffWorkflow->run()
#12 {main}
```