[test] Skip pexpect-based lldb-mi tests on Darwin

These tests fail with a relatively frequently on Darwin machines with
errors such as:

  File ".../lldb/third_party/Python/module/pexpect-2.4/pexpect.py", line 1444, in expect_loop
    raise EOF(str(e) + '\n' + str(self))
EOF: End Of File (EOF) in read_nonblocking(). Empty string style platform.

The unpredictable failures make these tests noisy.

rdar://37046976

llvm-svn: 326739