test/test-functions: instmods call to find should use -type f

without using -type f, the logs print an error such as:

E:  E: modprobe: FATAL: Module asymmetric_keys not found in directory /lib/modules/4.15.0-54-generic

while this doesn't appear to cause problems, it can be extremely
distracting when trying to debug real failures.

test/test-functions

@@ -1587,7 +1587,7 @@ instmods() {
                     | instmods
                 else
                     ( [[ "$_mpargs" ]] && echo $_mpargs
-                      find "$KERNEL_MODS" -path "*/${_mod#=}/*" -printf '%f\n' ) \
+                      find "$KERNEL_MODS" -path "*/${_mod#=}/*" -type f -printf '%f\n' ) \
                     | instmods
                 fi
                 ;;