Add test for mixing hip and openmp when using xnack (usm mode).

[carlobertolli]

trafficstars

Add test for USM mode when mixing hip and openmp but only openmp requires hw support for xnack

[zjin-lcf]

Is the following error expected on gfx908 ? Thanks.

HSA_XNACK=1 ./vecadd WARNING: USM SET WITHOUT XNACK ENABLED. THIS WILL BECOME FATAL ERROR IN FUTURE. Calling HIP

Calling OMP [GPU Memory Error] Addr: 0x1f36000 Reason: Page not present or supervisor privilege. Memory access fault by GPU node-2 (Agent handle: 0x1e32800) on address 0x1f36000. Reason: Page not present or supervisor privilege. Aborted