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.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2