[LibOS] test/ltp: Make sure we add all results to XML

An early return caused the runner to skip writing the test cases
that passed, but had no 'must-pass' list specified. This seems
unintentional and caused the runner to lose information about
results.

LibOS/shim/test/ltp/runltp_xml.py

@@ -313,12 +313,10 @@ class TestRunner:
                 returncode = await self._run_cmd()
 
             must_pass = self.cfgsection.getintset('must-pass')
-            if must_pass is None:
-                if returncode != 0:
-                    raise Fail('returncode={}'.format(returncode))
-                return
-
-            self._parse_test_output(must_pass)
+            if must_pass is not None:
+                self._parse_test_output(must_pass)
+            elif returncode != 0:
+                raise Fail('returncode={}'.format(returncode))
 
         except AbnormalTestResult as result:
             result.apply_to(self)