[lldb] fix fd leak during lldb testsuite

During lldb testing dotest.py opens files to dump testcase results, but
doesn't close them.

This patch makes neccessary changes to fix the file descriptors leak.

Author: daniilavdeev

lldb/packages/Python/lldbsuite/test/lldbtest.py

@@ -865,13 +865,9 @@ def setUp(self):
         session_file = self.getLogBasenameForCurrentTest() + ".log"
         self.log_files.append(session_file)
 
-        # Python 3 doesn't support unbuffered I/O in text mode.  Open buffered.
-        self.session = encoded_file.open(session_file, "utf-8", mode="w")
-
         # Optimistically set __errored__, __failed__, __expected__ to False
         # initially.  If the test errored/failed, the session info
-        # (self.session) is then dumped into a session specific file for
-        # diagnosis.
+        # is then dumped into a session specific file for diagnosis.
         self.__cleanup_errored__ = False
         self.__errored__ = False
         self.__failed__ = False
@@ -1235,20 +1231,25 @@ def dumpSessionInfo(self):
         else:
             prefix = "Success"
 
+        session_file = self.getLogBasenameForCurrentTest() + ".log"
+
+        # Python 3 doesn't support unbuffered I/O in text mode.  Open buffered.
+        session = encoded_file.open(session_file, "utf-8", mode="w")
+
         if not self.__unexpected__ and not self.__skipped__:
             for test, traceback in pairs:
                 if test is self:
-                    print(traceback, file=self.session)
+                    print(traceback, file=session)
 
         import datetime
 
         print(
             "Session info generated @",
             datetime.datetime.now().ctime(),
-            file=self.session,
+            file=session,
         )
-        self.session.close()
-        del self.session
+        session.close()
+        del session
 
         # process the log files
         if prefix != "Success" or lldbtest_config.log_success: