autotest: silence errors about missing timing.txt

Currently autotest prints the following at the start
of each run:

> grep: /[...]/output/timing.txt: No such file or directory

No harm is done and the file is created right after
the grep, but the message might be misleading. We get
rid of it now by ensuring the file always exists before
grepping it.

Change-Id: I271b68479a6fddcbe098c2657fd7c4dc39e6e50a
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/454708
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

test/common/autotest_common.sh

@@ -199,6 +199,7 @@ function timing() {
 		export timing_stack="${timing_stack};${now}"
 		export test_stack="${test_stack};${testname}"
 	else
+		touch "$output_dir/timing.txt"
 		child_time=$(grep "^${test_stack:1};" $output_dir/timing.txt | awk '{s+=$2} END {print s}')
 
 		start_time=$(echo "$timing_stack" | sed -e 's@^.*;@@')