selftests/ftrace: Fix checkbashisms errors

Fix a test case to make checkbashisms clean.
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/ftrace/test.d/trigger/trigger-trace-marker-snapshot.tc

@@ -39,10 +39,10 @@ test_trace() {
 	fi
 	echo "testing $line for >$x<"
 	match=`echo $line | sed -e "s/>$x<//"`
-	if [ "$line" == "$match" ]; then
+	if [ "$line" = "$match" ]; then
 	    fail "$line does not have >$x< in it"
 	fi
-	let x=$x+2
+	x=$((x+2))
     done
 }