Clarify bash (4?) requirement

[bgoglin]

We implicitly require bash on Linux for some tests (eg several test-topology.sh and even bash 4 in test-bash-completion.sh). It's configure-checked but those tests are never disabled if bash is missing.

[update 2024/07/29] As of 705a0369b7b0cbedf650f08697e4bb6bb6170653 checkbashism only finds $RANDOM as a bashism in our scripts. It's only used when mktemp isn't available, things should still work unless multiple tests are parallelized.

Some of these tests still use sh are shebang, they should be fixed. Or if we remove $RANDOM, we should switch all them from bash to sh.

Bash 4 is very old, RHEL 6 and Debian 6 already had it, hence requiring (for completion only?) should be fine.