[lldb] Fix PExpectTest when using utf-8 as the encoding

PExpect 4.6, when using 'utf-8' throws a TypeError when trying to write
to the log file:

  File "llvm-project/lldb/third_party/Python/module/pexpect-4.6/pexpect/spawnbase.py", line 126, in _log
    self.logfile.write(s)
  TypeError: a bytes-like object is required, not 'str'

This looks like a bug in PExpect to me. Since the log file is only used
for tracing, work around the issue by disabling the log file when
specifying an encoding. This should fix the Debian bot [1] which runs
the test with tracing enabled (-t).

[1] https://lab.llvm.org/buildbot/#/builders/68/builds/59955