ci: test Valgrind

ci: test Valgrind

Using `--trace-children=yes` has a trade-off here, as it makes
`test_scripts.sh` pretty slow when calling various non-xz utilities.

But I also feel like it's not useless to have Valgrind used there and it's
not easy to exclude Valgrind just for that one test...

I did consider using AX_VALGRIND_CHECK [0][1] but I couldn't get it working
immediately with some conditionally-built tests and I wondered if it was
worth spending time on at least while we're debating xz's future build
system situation.

[0] https://www.gnu.org/software/autoconf-archive/ax_valgrind_check.html
[1] https://tecnocode.co.uk/2014/12/23/automatically-valgrinding-code-with-ax_valgrind_check/