Update the fuzzcheck test module so that it avoids inserting text values

Update the fuzzcheck test module so that it avoids inserting text values
that contain embedded NULs in the XSQL table.  Fix some legacy entries in
the test/fuzzdata8.db that had embedded NULs.  Add in new dbsqlfuzz cases
that have accumulated over on the dbsqlfuzz project for a while.

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