Split unittest tests into separate test files.

pkg/pkg.status

Index: pkg/pkg.status
===================================================================
--- pkg/pkg.status	(revision 25630)
+++ pkg/pkg.status	(working copy)
@@ -20,10 +20,10 @@
 watcher/test/directory_watcher_test: Pass, Fail # Issue 12107
 
 [ $compiler == dart2js && $runtime == ff ]
-unittest/test/unittest_test: Pass, Timeout # http://dartbug.com/11473
+unittest/test/unittest_*_test: Pass, Timeout # http://dartbug.com/11473
 
 [ $runtime == d8 || $runtime == jsshell ]
-unittest/test/unittest_test: Pass, Fail # http://dartbug.com/10109
+unittest/test/unittest_nested_groups_setup_teardown_test: Pass, Fail # http://dartbug.com/10109
 stack_trace/test/vm_test: Fail, OK # VM-specific traces
 sequence_zip/test/stream_test: Fail, OK # Timers are not supported.
 
@@ -71,7 +71,7 @@
 unittest/test/mock_stepwise_negative_test: Skip
 
 [ $compiler == dart2js && $csp ]
-unittest/test/unittest_test: Pass, Crash # Issue 10935
+unittest/test/unittest_*_test: Pass, Crash # Issue 10935
 observe/test/observe_test: Fail # Issue 11970
 observe/test/path_observer_test: Fail  # Issue 11970
 mdv/test/binding_syntax_test: Fail # Issue 11970