test/py: allow multi-digit index in in_tree()

When more nodes are added for a uclass the index might go into two or
more digits. This means that there are less spaces printed because they
are used up by the extra digits. Update the regular expression to allow
variable-length spacing between the class name and and index.

This was discovered when adding a simple_bus node in test.dts made
test_bind_unbind_with_uclass() fail because the index went up to 10.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>