Fluxion Valgrind tests aren't run

[milroy]

The migration from autotools to cmake has resulted in Valgrind tests not being run. I discovered this while working on PR #1061. I realized that a few changes to planner introduced memory leaks that weren't caught by t5000-valgrind.t. Upon examination, have_valgrind_h is false

flux-sched/t/t5000-valgrind.t

Line 33 in 7b1a79f

if ! have_valgrind_h && test "$debug" = ""; then

and the test is skipped. We need to add a FindValgrind.cmake to detect the valgrind location and set HAVE_VALGRIND, updating t5000-valgrind.t appropriately.