tests: use bash in some scripts to avoid false failures

tests: use bash in some scripts to avoid false failures

* init.cfg (require_bash_as_SHELL_): A new function to replace
SHELL for the current test, with bash if available.
This is useful on OpenIndiana 11 where /bin/sh was seen
to have races in handling of SIGPIPE.
* tests/misc/seq-epipe.sh: Use the new function to enforce bash.
* tests/misc/env-signal-handler.sh: Likewise.
Reported by Bruno Haible