testsuite: check CONFIG_DESKTOP before using 'od -t'

The '-t' option for od is enabled by CONFIG_DESKTOP. So
check this config before using 'od -t' in test cases.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/testsuite/echo/echo-prints-slash_00041 b/testsuite/echo/echo-prints-slash_00041
index 9cffabd..7ea217b 100644
--- a/testsuite/echo/echo-prints-slash_00041
+++ b/testsuite/echo/echo-prints-slash_00041
@@ -1,3 +1,3 @@
-# FEATURE: CONFIG_FEATURE_FANCY_ECHO
+# FEATURE: CONFIG_FEATURE_FANCY_ECHO CONFIG_DESKTOP
 
 test "`busybox echo -ne '\00041z' | od -t x1 | head -n 1`" = "0000000 04 31 7a"