[SYCL] Enable fallback assertion test for OpenCL backend (#1026)

Author: againull

SYCL/Assert/assert_in_simultaneously_multiple_tus.cpp

@@ -1,9 +1,6 @@
 // FIXME unsupported on CUDA and HIP until fallback libdevice becomes available
-// UNSUPPORTED: cuda || hip
-// clang-format off
-// Failed on Linux on unrelated change (FileCheck error: '.../assert_in_simultaneously_multiple_tus.cpp.tmp.txt' is empty)
-// clang-format on
-// REQUIRES: TEMPORARILY_DISABLED
+// FIXME flaky output on Level Zero
+// UNSUPPORTED: cuda || hip || level_zero
 // RUN: %clangxx -DSYCL_FALLBACK_ASSERT=1 -fsycl -fsycl-targets=%sycl_triple -I %S/Inputs %s %S/Inputs/kernels_in_file2.cpp -o %t.out %threads_lib
 // RUN: %CPU_RUN_PLACEHOLDER %t.out &> %t.txt || true
 // RUN: %CPU_RUN_PLACEHOLDER FileCheck %s --input-file %t.txt