tdb: test that new-style TDBs with non-default hashes can't be opened.

We currently allow opening of a TDB without any idea if we're using the
right hash function.  Since most people use the default, this hasn't been
a big issue.

The next change will put an example hash value in the rwlocks field in
the header: current TDB already refuses to open a TDB with this field
non-zero.  This commit simply adds tests that that occurs.