Test failures and stream leaking in test_test_protocol suite

Running subunit.tests.test_test_protocol.test_suite under testtools results on my machine in four platform related failures and some extra junk on stdout. With Robert's py3k branch, these failures are also present in slightly different forms under Python 3. The issues are:

Should be to be skipped on platforms without fork:
* TestIsolatedTestCase.test_run
* TestIsolatedTestSuite.test_run

Probably broken by not using binary streams in subprocess:
* TestExecTestCase.test_run

Needs to be aware of os.sep and relative paths:
* TestExecTestCase.test_join_dir

Also, there are tests that result in spew to stdout:
* TestTestProtocolServerStartTest.test_indented_test_colon_ignored
* TestExecTestCase.test_run
* TestExecTestCase.test_debug