fts3 functions for testing scripts. These are a first step towards

fts3 functions for testing scripts.  These are a first step towards
being able to write test script which verify that fts3 is internally
building indices in the expected way.  Both new functions are only
defined if fts3.c is compiled with SQLITE_TEST defined, as when
building testfixture.  These functions are not intended to be part of
the exposed fts3 API.

dump_terms() generates a TEXT result of all the terms in the index (or
a specified segment), sorted and joined with spaces.

dump_doclist() generates a TEXT representation of the doclist
associated with a given term in the index (or a specified segment). (CVS 5340)

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