[OpenMP][OMPT] Expect failure from tool_available_search.c on macOS

D91464 introduced verbose tool loading, but the test check only considers Linux.
On macOS, the outputs are totally different, causing the regression afterwards.
This patch simply sets the test to XFAIL on macOS.

Fix #56833.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D142045