tests: shunit2: add test for _jshn_append via JSON script

tests: shunit2: add test for _jshn_append via JSON script

Add another test which verifies _jshn_append leading space fix in commit
82cb5fd66af9 ("libubox: Drop extraneous space when appending values to
variable") by appending keys to JSON objects, making sure there is no
leading space on the first key when adding second key.

  test_jshn_append_via_json_script
  ASSERT:expected:<first> but was:< first>
  ASSERT:expected:<first second> but was:< first second>

Tests: #16
Signed-off-by: Petr Štetiar <[email protected]>
Link: https://github.com/openwrt/libubox/pull/30
Signed-off-by: Álvaro Fernández Rojas <[email protected]>