Flag lib link: Use test variant when building test

This focus on net_test_bluetooth, net_test_bta, net_test_avrcp because
they gave me trouble to pass

Bug: 311772251
Test: mmm p/m/Bluetooth
Flag: Exempt build change
Change-Id: I68fc37a6e1896d6ba1010da1d965f0de3e8b1641