meson: don't require valgrind tests

meson: don't require valgrind tests

It's entirely possible some people don't have valgrind installed, but
still want to run the tests. If they don't have it installed, then they
probably don't intend to run those precise test targets anyway.

Also, this solves an error when running the tests in an automated
environment. The valgrind tests have a hard dependency on behavior such
as `./zstd` erroring out with the message "stdin is a console, aborting"
which does not work if the automated environment doesn't have a console.
As a rough heuristic, automated environments lacking a console will
*probably* also not have valgrind, so avoiding that test definition
neatly sidesteps the issue.

Also, valgrind is not easily installable on macOS, at least homebrew
says it isn't available there. This makes it needlessly hard to
enable the testsuite on macOS.