Implement the magic-cookie check and its test.

tests/badmagic.chk

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+
+Welcome to Adventure!!  Would you like instructions?
+
+> n
+
+You are standing at the end of a road before a small brick building.
+Around you is a forest.  A small stream flows out of the building and
+down a gully.
+
+> resume
+Can't open file y, try again.
+
+Oops, that does not look like a valid save file.
+
+You're in front of building.
+
+> 
+You scored 32 out of a possible 430, using 1 turn.
+
+You are obviously a rank amateur.  Better luck next time.
+
+To achieve the next higher rating, you need 14 more points.

tests/badmagic.log

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+## Resume from filename withoy the right magic at the front 
+# SPDX-FileCopyrightText: Eric S. Raymond <esr@thyrsus.com>
+# SPDX-License-Identifier: BSD-2-Clause
+#NOCOMPARE advent430 doesn't have this test
+n
+resume
+y
+../main.o