tests: shunit2: add test for _jshn_append via JSON script

[ynezz]

tests: shunit2: add test for _jshn_append via JSON script

Add another test which verifies _jshn_append leading space fix in commit
82cb5fd ("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

tests: shunit2: add bash shebang to fix export -n error in dash

The test script was missing a shebang, causing it to be executed with
/bin/sh (often dash on Debian/Ubuntu systems). When the test sources
jshn.sh, which uses 'export -n', dash fails with:

  libubox/tests/shunit2/tests.sh: 125: export: Illegal option -n

Fix it by adding bash shebang.

---

https://github.com/ynezz/openwrt-libubox/actions/runs/19331703870/job/55296135170#step:9:34

Test results with 82cb5fd fix reverted

Run make test CTEST_OUTPUT_ON_FAILURE=1
Running tests...
Test project /home/runner/work/openwrt-libubox/openwrt-libubox
    Start 1: cram
1/2 Test #1: cram .............................   Passed   45.61 sec
    Start 2: shunit2
2/2 Test #2: shunit2 ..........................***Failed    0.20 sec
#
# Performing tests
#
test_bad_json
test_expr_eq
test_expr_has
test_expr_regex_single
test_expr_regex_multi
test_expr_not
test_expr_and
test_expr_or
test_expr_isdir
test_cmd_case
test_cmd_if
test_cmd_cb
test_cmd_return
test_jshn_append_no_leading_space
ASSERT:expected:<foo> but was:< foo>
ASSERT:expected:<first second> but was:< first second>
test_jshn_append_via_json_script
ASSERT:expected:<first> but was:< first>
ASSERT:expected:<first second> but was:< first second>

#
# Test report
#
tests passed:     1  20%
tests failed:     4  80%
tests skipped:    0   0%
tests total:      5 100%

Errors while running CTest

50% tests passed, 1 tests failed out of 2

Total Test time (real) =  45.81 sec

The following tests FAILED:
	  2 - shunit2 (Failed)
make: *** [Makefile:71: test] Error 8

[l-jonas]

This adds tests for the manually merged #16. Now, one can easily see why that other PR is useful. Now I see a reason for merging that other PR and this one too.