Do not expect a nothing-to-do migration to fail on a read-only DB file

--- mtn-src/test/func/fail_cleanly_on_unreadable_db/__driver__.lua	2026-08-05 21:29:54.727038442 +0200
+++ mtn-src/test/func/fail_cleanly_on_unreadable_db/__driver__.lua	2026-08-05 21:30:33.397412828 +0200
@@ -23,7 +23,7 @@
 check(mtn("ls", "branches"), 0, false, false)
 check(mtn("db", "info"), 0, false, false)
 check(mtn("db", "version"), 0, false, false)
-check(mtn("db", "migrate"), 1, false, false)
+check(mtn("db", "migrate"), 0, false, false) -- A migration is a NOP, RO is OK
 check(mtn("commit", "-mfoo"), 1, false, false)
 check(mtn("db", "load"), 1, false, false)
 check({"chmod", "a+w", "test.db"})