flux-sharness: add env vars for heaptrack and wrap

problem: I've been using heaptrack a lot, and other tools that need to
be wrapped around flux itself.  This is difficult without something to
integrate with test_funder_flux

solution: add some env vars to do a heaptrack recording or add a generic
wrapper

Author: trws

t/sharness.d/flux-sharness.sh

@@ -253,6 +253,10 @@ test_under_flux() {
         valgrind="$valgrind,--trace-children=no,--child-silent-after-fork=yes"
         valgrind="$valgrind,--leak-resolution=med,--error-exitcode=1"
         valgrind="$valgrind,--suppressions=${VALGRIND_SUPPRESSIONS}"
+    elif test -n "$FLUX_TEST_HEAPTRACK" ; then
+        valgrind="--wrap=heaptrack,--record-only"
+    elif test -n "$FLUX_TEST_WRAP" ; then
+        valgrind="$FLUX_TEST_WRAP"
     fi
     # Extend timeouts when running under AddressSanitizer
     if test_have_prereq ASAN; then