t-reftable-tree: split test_tree() into two sub-test functions

t-reftable-tree: split test_tree() into two sub-test functions

In the current testing setup, tests for both tree_search() and
infix_walk() defined by reftable/tree.{c, h} are performed by
a single test function, test_tree(). Split tree_test() into
test_tree_search() and test_infix_walk() responsible for
independently testing tree_search() and infix_walk() respectively.
This improves the overall readability of the test file as well as
simplifies debugging.

Note that the last parameter in the tree_search() functiom is
'int insert' which when set, inserts the key if it is not found
in the tree. Otherwise, the function returns NULL for such cases.

While at it, use 'func' to pass function pointers and not '&func'.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>