tests: shell: add JSON test for handle-based rule positioning

tests: shell: add JSON test for handle-based rule positioning

Add comprehensive test for JSON handle-based rule positioning to verify
the handle field correctly positions rules with explicit add/insert
commands while being ignored in implicit format.

Test coverage:
1. ADD with handle positions AFTER the specified handle
2. INSERT with handle positions BEFORE the specified handle
3. INSERT without handle positions at beginning
4. Multiple commands in single transaction (batch behavior)
5. Implicit format ignores handle field for portability

The test uses sed for handle extraction and nft -f format for setup
as suggested in code review. Final state is a table with two rules
from the implicit format test.

Signed-off-by: Alexandre Knecht <knecht.alexandre@gmail.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>