hghave: add test for clang 6 and later clang 6 includes integrated libfuzzer support, which makes fuzzer stuff a little easier to work with. Differential Revision: https://phab.mercurial-scm.org/D1873

py3: cast error message to localstr in blackbox.py According to the exceptions reporting mechanism, this is the #1 crasher in the test harness for Python 3. Since this exception is in an except block, we'll likely get a new #1 crasher after this change. But at least we won't die during error handling! Differential Revision: https://phab.mercurial-scm.org/D1478