Run bulk 'make valgrind' in fast mode

Use the new support for --valgrind=memcheck-fast to speed up bulk
valgrind runs, where we expect the user to only be interested in which
test(s) fail, if any.  The speedup is easily worth manually re-running
a more thorough memcheck to investigate any issues found.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>