[string] How do you append to an already existing string?

I want append to a string so that every time I loop over it will add say "test" to the string.

Like in PHP you would do:

$teststr = "test1\n"
$teststr .= "test2\n"
echo = "$teststr"

echos:

test1
test2

But I need to do this in a shell script

VAR=$VAR"$VARTOADD(STRING)"   
echo $VAR

thank-you Ignacio Vazquez-Abrams

i adapted slightly for better ease of use :)

placed at top of script

NEW_LINE=$'\n'

then to use easily with other variables

variable1="test1"
variable2="test2"

DESCRIPTION="$variable1$NEW_LINE$variable2$NEW_LINE"

OR to append thank-you William Pursell

DESCRIPTION="$variable1$NEW_LINE"
DESCRIPTION+="$variable2$NEW_LINE"

echo "$DESCRIPTION"

#!/bin/bash
message="some text"
message="$message add some more"

echo $message

some text add some more

teststr=$'test1\n'
teststr+=$'test2\n'
echo "$teststr"

#!/bin/bash

msg1=${1} #First Parameter
msg2=${2} #Second Parameter

concatString=$msg1"$msg2" #Concatenated String
concatString2="$msg1$msg2"

echo $concatString 
echo $concatString2

$ string="test"
$ string="${string}test2"
$ echo $string
testtest2