Add more tests

Hopefully, this helps us catch more missing functionality.

.test/config.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+# https://github.com/docker-library/official-images/blob/618a85a87796c8bbea059011d94d888dc7554158/test/config.sh
+
+imageTests+=(
+	[bash]='bash-optional-features'
+)

.test/tests/bash-optional-features/run.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+# there's a lot of commands we expect Bash to have, but that it might not due to compilation quirks
+# see https://github.com/tianon/docker-bash/pull/45
+# and https://www.gnu.org/software/bash/manual/html_node/Optional-Features.html#:~:text=unless%20the%20operating%20system%20does%20not%20provide%20the%20necessary%20support
+
+image="$1"
+
+args=( --rm --interactive )
+if [ -t 0 ] && [ -t 1 ]; then
+	args+=( --tty )
+fi
+
+docker run "${args[@]}" "$image" -Eeuo pipefail -xc '
+	# --enable-array-variables 😏
+	cmds=(
+		# --enable-alias
+		alias unalias
+		# --enable-command-timing
+		time
+		# --enable-cond-command
+		"[["
+		# --enable-coprocesses
+		coproc
+		# --enable-directory-stack
+		pushd popd dirs
+		# --enable-disabled-builtins
+		builtin enable
+		# --enable-help-builtin
+		help
+		# --enable-history
+		fc history
+		# --enable-job-control
+		bg fg jobs kill wait disown suspend
+		# --enable-progcomp
+		complete
+		# --enable-select
+		select
+	)
+	for cmd in "${cmds[@]}"; do
+		PATH= command -v "$cmd"
+	done
+'