test: Make the --valgrind option useful, (and drop --verbose).

test: Make the --valgrind option useful, (and drop --verbose).

In order for --valgrind to be useful, we drop noisy additional output of
all of the commands being executed in verbose mode. This makes --verbose
alone quite useless, so we don't document it any more.

Also, add a zlib valgrind suppression that was showing up frequently in the
test suite.