Fix the hidden main function in VTS tests

Otherwise gtest will inherit the one from libgtest_main which
will not start thread pool to handle binder call for callbacks

Bug: b/205762943
Test: atest VtsHalThermalTargetTest VtsHalPowerTargetTest VtsHalTetheroffloadTargetTest
Change-Id: Ib9e5d265b8f5081dd20568a98d01e9190177440a
3 files changed