The * is expanded, what you can do is use sed instead of grep and get the name of the branch immediately:
branch=$(git branch | sed -n -e 's/^\* \(.*\)/\1/p')
And a version using git symbolic-ref, as suggested by Noufal Ibrahim
branch=$(git symbolic-ref HEAD | sed -e 's,.*/\(.*\),\1,')
To elaborate on the expansion, (as marco already did,) the expansion happens in the echo, when you do echo $test with $test containing * master then the * is expanded according to the normal expansion rules. To suppress this one would have to quote the variable, as shown by marco: echo "$test". Alternatively, if you get rid of the asterisk before you echo it, all will be fine, e.g. echo ${test:2} will just echo master. Alternatively you could assign it anew as you already proposed:
branch=${test:2}
echo $branch
This will echo master, like you wanted.