[lit, lldbsuite] Fixes for several tests LLDB tests for Python 3 or Windows

Summary:
In decorators.py, when opening streams, open them in text mode. In Py3, if they are not opened in text mode, the data is also expected to be binary, but we always use text data.
In TestLinuxCore, skip the tests that are not applicable on Windows
In the python api main.c, update the code to be compilable on Windows

Reviewers: asmith, zturner

Reviewed By: zturner

Subscribers: zturner

Differential Revision: https://reviews.llvm.org/D46440

llvm-svn: 331686