tests: let mkdir/p-v.sh fail when compare fails

* tests/mkdir/p-v.sh: Set fail=1 when compare fails: the latter only
sets a non-Zero return code; evaluating that is up to the caller.

tests/mkdir/p-v.sh

@@ -21,7 +21,7 @@ print_ver_ mkdir
 
 mkdir -pv foo/a/b/c/d >out || fail=1
 
-compare - out <<\EOF
+compare - out <<\EOF || fail=1
 mkdir: created directory 'foo'
 mkdir: created directory 'foo/a'
 mkdir: created directory 'foo/a/b'