[libcxx] [test] Read files as bytestrings to fix py3 encoding issues

Use binary mode to read test files in libcxx LibcxxTestFormat class.
This ensures that tests are read correctly independently of encoding,
and therefore fixes UnicodeDecodeError when file is opened in Python 3
that defaults to pure ASCII encoding.

Technically this could be also fixed via conditionally appending
encoding argument when opening the file in Python 3.  However, since
the code in question only searches for fixed ASCII substrings reading
it in binary mode is simpler and more universal.

Differential Revision: https://reviews.llvm.org/D63346

llvm-svn: 364170