[lldb] Skip TestChangeProcessGroup on watchOS/tvOS

[teemperor]

Summary

fork is marked as __WATCHOS_PROHIBITED __TVOS_PROHIBITED so the test source
which is calling fork will never compile on watchOS/tvOS. This just adds the skip decorator for these platforms.

[mib]

LGTM!

lldb/test/API/functionalities/process_group/TestChangeProcessGroup.py

def setUp(self):
         # Find the line number to break for main.c.
         self.line = line_number('main.c', '// Set breakpoint here')
 
     @skipIfFreeBSD  # Times out on FreeBSD llvm.org/pr23731
     @skipIfWindows  # setpgid call does not exist on Windows
     @expectedFailureAndroid("http://llvm.org/pr23762", api_levels=[16])
     @expectedFailureNetBSD
     @skipIfReproducer # File synchronization is not supported during replay.
+    @skipIftvOS # fork not available on tvOS.
+    @skipIfwatchOS # fork not available on watchOS.
     def test_setpgid(self):
         self.build()
         exe = self.getBuildArtifact("a.out")
 
         # Use a file as a synchronization point between test and inferior.
         pid_file_path = lldbutil.append_to_process_working_directory(self,
             "pid_file_%d" % (int(time.time())))
         self.addTearDownHook(