Run test suite under valgrind

[bnoordhuis]

Valgrind catches bugs that {A,M,UB}San won't catch. The downside is it's sloooow and maybe not something we want to run for every commit.

Test how long it takes on GHA. If it's beyond reasonable, then run it daily as a GHA cron job?

[saghul]

Would you run the entire 262 suite or a subset?

[bnoordhuis]

The whole suite.

[saghul]

I'm trying this locally and it's unbearably slow, let's see how it does in the CI: #324 at least it seems to have caught something... #323