Recursive btree traversal test case

Recursive btree traversal test case

Add an unlink page command to the dbtest program.  This dbtest command
finds a page that has both a left and a right neighbor and unlinks it,
making it inaccessible to conventional sequential traversal.  This
simulates some btree corruption that has been seen in the field.

Unlike the bttest command, the dbtest unlink command always searches
for a leaf page with both a left and a right sibling, and doesn't
allow the user to specify internal pages or a specific page number.

Add a new dbtest command to recursively dump a btree database.

Add a new test case to run.test that uses these new commands to verify
the correct functioning of the recursive btree traversal options.

ticket: 8476