[SYCL] Prevent skipped GPU run from overwriting CPU output in DeviceLib/assert.cpp (intel#1228)

Though the two potential runs in DeviceLib/assert.cpp are mutually
exclusive, skipping the latter run (GPU) will still trigger a write
to the output file, overwriting the results from the previous (CPU) run.
To avoid this, these changes first ensures the output file is cleared
then makes the runs append their output to the file. Since the runs are
mutually exclusive the file should at most contain the output of one
expected run.

Signed-off-by: Larsen, Steffen <steffen.larsen@intel.com>

Author: steffenlarsen

SYCL/DeviceLib/assert.cpp

@@ -71,8 +71,12 @@
 //
 // Overall this sounds stable enough. What could possibly go wrong?
 //
-// RUN: %CPU_RUN_PLACEHOLDER env SYCL_PI_TRACE=2 SHOULD_CRASH=1 EXPECTED_SIGNAL=SIGABRT %t.out 2>%t.stderr.native
-// RUN: %GPU_RUN_PLACEHOLDER env SHOULD_CRASH=1 EXPECTED_SIGNAL=SIGIOT %t.out 2>%t.stderr.native
+// With either a CPU run or a GPU run we reset the output file and append the
+// results of the runs. Otherwise a skipped GPU run may remove the output from
+// a CPU run prior to running FileCheck.
+// RUN: echo "" > %t.stderr.native
+// RUN: %CPU_RUN_PLACEHOLDER SYCL_PI_TRACE=2 SHOULD_CRASH=1 EXPECTED_SIGNAL=SIGABRT %t.out 2>> %t.stderr.native
+// RUN: %GPU_RUN_PLACEHOLDER SHOULD_CRASH=1 EXPECTED_SIGNAL=SIGIOT %t.out 2>> %t.stderr.native
 // RUN: FileCheck %s --input-file %t.stderr.native --check-prefixes=CHECK-MESSAGE || FileCheck %s --input-file %t.stderr.native --check-prefix CHECK-NOTSUPPORTED
 //
 // Skip the test if the CPU RT doesn't support the extension yet: