TEST: 'keyctl invalidate' exists from keyutils-1.5.6, not keyutils-3.5-rc1

Fix mixed up version number checking.  3.5-rc1 is a kernel version number, not
a keyutils one.

Signed-off-by: David Howells <dhowells@redhat.com>

tests/prepare.inc.sh

diff --git a/tests/prepare.inc.sh b/tests/prepare.inc.sh
index ee09775..bbbdcba 100644
--- a/tests/prepare.inc.sh
+++ b/tests/prepare.inc.sh
@@ -73,7 +73,7 @@
 # Work out whether key invalidation is supported by the kernel
 #
 have_key_invalidate=0
-if keyutils_at_or_later_than 3.5-rc1 && kernel_at_or_later_than 3.5-rc1
+if keyutils_at_or_later_than 1.5.6 && kernel_at_or_later_than 3.5-rc1
 then
     have_key_invalidate=1
 fi