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.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL