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

init.cfg

@@ -629,6 +629,15 @@ trap_sigpipe_or_skip_()
     skip_ 'trapping SIGPIPE is not supported'
 }
 
+require_bash_as_SHELL_()
+{
+  if bash --version >/dev/null 2>&1; then
+    SHELL='bash'
+  else
+    skip_ 'bash is required'
+  fi
+}
+
 # Disable the current test if the working directory seems to have
 # the setgid bit set.
 skip_if_setgid_()