In Bash if you export the variable within a subshell, using parentheses as shown, you avoid leaking the exported variables:
#!/bin/bash
TESTVARIABLE=hellohelloheloo
(
export TESTVARIABLE
source ./test2.sh
)
The advantage here is that after you run the script from the command line, you won't see a $TESTVARIABLE leaked into your environment:
$ ./test.sh
hellohelloheloo
$ echo $TESTVARIABLE
#empty! no leak
$