[lldb][NFC] enable unwind logging in TestCortexMExceptionUnwind

Still seeing a failure on a CI bot with this test that
I cannot reproduce locally, but luckily this one CI bot is giving
me trace output from the tests.  Turn on the unwind log when tracing
is enabled, migth get a better hint what's up with this test fail.

Author: jasonmolenda

lldb/test/API/functionalities/unwind/cortex-m-exception/TestCortexMExceptionUnwind.py

@@ -28,6 +28,9 @@ def test_no_fpu(self):
         core = self.getBuildArtifact("core")
         self.yaml2macho_core("armv7m-nofpu-exception.yaml", core, exe_uuid)
 
+        if self.TraceOn():
+            self.runCmd("log enable lldb unwind")
+
         process = target.LoadCore(core)
         self.assertTrue(process.IsValid())