This error is caused because Switfmailer defines a constant SWIFT_REQUIRED_LOADED after it installs it's autoloader. The autoloader checks this constant, and if it's already defined, refuses to install the autoloader. In process isolation, PHPUnit ensures that all defined constants are re-defined in the processs that gets spawned to run the test. Unfortunately, this means that SWIFT_REQUIRED_LOADED is defined in the test process, so the autoloader doesn't load (see "swift_required.php" in the swiftmailer source directory). Note that if you turn off include global state for the test via annotations your tests still won't work, because the bootstrap file is passed to the test process via the __PHPUNIT_BOOTSTRAP global variable (see TestCaseMethod.tpl in the PHPUnit directory). Without the globals, this global is undefined in the test process and the bootstrap file is not included breaking the tests.
The only work around that I found that works was to replace the line
$constants = PHPUnit_Util_GlobalState::getConstantsAsString();
with
$constants = '';
in the run method of TestCase.php in the PHPUnit source distribution. If your code
relies on global constants to be defined before your test cases run
this fix obviously won't work for you (class constants are a
different story); unless said constants will be redefined safely in the
course of running your test cases.