[lldb] Improve error logging in test (NFC)

Author: adrian-prantl

lldb/test/API/macosx/posix_spawn/TestLaunchProcessPosixSpawn.py

@@ -36,9 +36,11 @@ def no_apple_silicon(self):
 
     def run_arch(self, exe, arch):
         self.runCmd("target create -arch {} {}".format(arch, exe))
-        self.runCmd("run")
-
-        process = self.dbg.GetSelectedTarget().process
+        target = self.dbg.GetSelectedTarget()
+        launch_info = target.GetLaunchInfo()
+        error = lldb.SBError()
+        process = target.Launch(launch_info, error)
+        self.assertTrue(error.Success, str(error))
         self.assertState(process.GetState(), lldb.eStateExited)
         self.assertIn("slice: {}".format(arch), process.GetSTDOUT(1000))