[lldb/Test] Add a trace method to replace print statements.

Many tests use (commented out) print statement for debugging the test
itself. This patch adds a new trace method to lldbtest to reuse the
existing tracing infrastructure and replace these print statements.

Differential revision: https://reviews.llvm.org/D80448

lldb/test/API/python_api/thread/TestThreadAPI.py

@@ -100,7 +100,7 @@ class ThreadAPITestCase(TestBase):
         self.runCmd("process status")
 
         proc_of_thread = thread.GetProcess()
-        #print("proc_of_thread:", proc_of_thread)
+        self.trace("proc_of_thread:", proc_of_thread)
         self.assertTrue(proc_of_thread.GetProcessID()
                         == process.GetProcessID())