tests/narrow-library.sh

test-fuzz-targets: look for clang-6.0 binary as well

Debian sid is still shipped with clang 4.0 by default. This allows me to
run the test without replacing the system clang to clang-6.0.

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF