bash: Use all elements of all arrays in one for-clause; see https://forum.ubuntuusers.de/topic/verarbeitung-von-arrays/

reg.txt

TEST_A=(
        '1a'
        '2a'
        '3a'
)

TEST_B=(
        '1b'
        '2b'
        '3b'
)

x.sh

# Nr. 1
source reg.txt

for a in "${!TEST@}"
do
        # eval 'A=("${'"${_a}"'[@]}")'
        typeset -a 'A=("${'"${a}"'[@]}")'
        for aa in "${A[@]}"
        do
                echo "$aa"
        done
done

# Nr. 2

source reg.txt

for a in "${!TEST@}"
do
        A="$a[@]"
        for aa in "${!A}"
        do
	        echo "$aa"
	done
done

Nr. 3

(geirha in bash@freenode):

source reg.txt
typeset -a "A=( $(printf '"${%s[@]}" ' "${!TEST_@}") )"
for a in "${A[@]}"
do
        echo "$a"
done