Converge ext/[GNOS]DBM_File/t/[gnos]dbm.t by using the same filename.

Converge ext/[GNOS]DBM_File/t/[gnos]dbm.t by using the same filename.

Choose the 1 dot form used by sdbm.t, to keep VMS happy. Also, propagate into
ndbm.t the part of the test for 20001013.009 that cbc5248d01a71061 missed.
Move the exist tests from f4b9d8806d76b352 earlier in sdbm.t, to increase
consistency - the alternative attempts to have 2 DBM files open simultaneously,
which ODBM_File doesn't support. (Implied TODO: add an explicit test for this
to the other 3.)