t2026 needs procondition SANITY

When running t0026 as root 'prune directories with unreadable gitdir' fails.
Skip this test if SANITY is not set (the use of POSIXPERM is wrong here)
Signed-off-by: NTorsten Bögershausen <tboegi@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t2026-prune-linked-checkouts.sh

@@ -33,7 +33,7 @@ EOF
 	! test -d .git/worktrees
 '
 
-test_expect_success POSIXPERM 'prune directories with unreadable gitdir' '
+test_expect_success SANITY 'prune directories with unreadable gitdir' '
 	mkdir -p .git/worktrees/def/abc &&
 	: >.git/worktrees/def/def &&
 	: >.git/worktrees/def/gitdir &&