Use magic rather than DESTROY to free memory in Storable's context.

Suggested by Leon Timmermans.
[perl #118139] Storable 2.42 - die in global destruction

Protect against SEGV during global destruction, e.g. when used
in DESTROY blocks.

[Part of a patch submitted by Reini. MANIFEST change added by the committer]
Tweak the new Storable destroy test, and the ChangeLog

Use Test::More instead of Test, as all the other tests now use Test::More.
Use 2-argument open to be compatible with 5.005. (Where it passes)
Close the test file before unlinking it.
Check the error returns of open and close.
Update the comment now that we have a better understanding of the bug's cause.

As the previous commit as applied had no 5.6 compatibility changes, remove
that note from the ChangeLog.