[LNT] Python 3 support: run tests with UTF-8 encoding

Summary:
Click requires to be invoked with a default encoding of UTF-8. If not,
it exists the program with an error. This commit sets the LANG
environment variable for running lit tests to the system locale if it is
a UTF-8 capable one or en_US.UTF-8 as a fallback. This allows the test
to run on systems where en_US.UTF-8 is not installed (such as some
container images) but the default locale is UTF-8 capable.

Reviewers: cmatthews, hubert.reinterpretcast, kristof.beyls, leandron, PrzemekWirkus

Reviewed By: hubert.reinterpretcast

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D69048

Author: Thomas Preud'homme

tests/lit.cfg

@@ -33,6 +33,10 @@ try:
 except:
     build_root = ''
 config.environment['PYTHONPATH'] = '%s:%s' % (build_root, src_root)
+# LNT needs a UTF-8 locale when running in Python 3 mode. Rely on the host
+# environment to provide it rather than assume a specific UTF-8 locale is
+# present.
+config.environment['LANG'] = os.environ['LANG']
 # Don't generate .pyc files when running tests.
 config.environment['PYTHONDONTWRITEBYTECODE'] = "1"
 config.environment['SUDO_CMD'] = ""