subtree: t7900: use test-lib.sh's test_count

Use test-lib.sh's `test_count`, instead instead of having
t7900-subtree.sh do its own book-keeping with `subtree_test_count` that
has to be explicitly incremented by calling `next_test`.

Signed-off-by: Luke Shumaker <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: LukeShu