Change BTS to GRAPHENE_SYMBOL for testnet cli_test

[abitmore]

Travis complains when running cli_test for testnet branch (link):

3364738ms th_a main.cpp:512 test_method ] e.to_detail_string(): 10 assert_exception: Assert Exception
a:
{}
th_a wallet.cpp:3213 get_asset
{"from_account_id_or_name":"nathan","asset_symbol":"BTS","to_amounts":[["nathan","100000000"]]}
th_a wallet.cpp:4488 transfer_to_blind

This PR changed hard coded BTS to GRAPHENE_SYMBOL in order to fix this.

[oxarbitrage]

make sense. thanks.