[Libomptarget] Disable testing map_back_race.cpp

This test hasn't been fixed and causes spurious failures when testing.
This patch sets it as unsupported until we have a reliable fix.

Reviewed By: ronlieb

Differential Revision: https://reviews.llvm.org/D130789

GitOrigin-RevId: 1d03b2efcd40eb8225a0b10afea86e8a1436681e
1 file changed