[lldb] [unittests] Disable MainLoopTest::DetectsEOF on NetBSD

The NetBSD kernel currently does not support detecting closed slave pty
via kevent on master pty.  This causes the test to hang forever.
To avoid that, disable the test until the kernel is fixed.

Differential Revision: https://reviews.llvm.org/D57912

llvm-svn: 353545