[lldb] Enable FreeBSDRemote plugin by default and update test status

The new FreeBSDRemote plugin has reached feature parity on i386
and amd64 targets.  Use it by default on these architectures, while
allowing the use of the legacy plugin via FREEBSD_LEGACY_PLUGIN envvar.

Revisit the method of switching plugins.  Apparently, the return value
of PlatformFreeBSD::CanDebugProcess() is what really decides whether
the legacy or the new plugin is used.

Update the test status.  Reenable the tests that were previously
disabled on FreeBSD and do not cause hangs or are irrelevant to FreeBSD.
Mark all tests that fail reliably as expectedFailure.  For now, tests
that are flaky (i.e. produce unstable results) are left enabled
and cause unpredictable test failures.

Differential Revision: https://reviews.llvm.org/D90757

lldb/test/API/functionalities/plugins/python_os_plugin/stepping_plugin_threads/TestOSPluginStepping.py

@@ -19,6 +19,7 @@ class TestOSPluginStepping(TestBase):
     NO_DEBUG_INFO_TESTCASE = True
 
     @skipIfWindows
+    @skipIfFreeBSD  # hangs
     def test_python_os_plugin(self):
         """Test that stepping works when the OS Plugin doesn't report all
            threads at every stop"""
@@ -27,6 +28,7 @@ class TestOSPluginStepping(TestBase):
         self.run_python_os_step_missing_thread(False)
 
     @skipIfWindows
+    @skipIfFreeBSD  # hangs
     def test_python_os_plugin_prune(self):
         """Test that pruning the unreported PlanStacks works"""
         self.build()