t5403: introduce check_post_checkout helper function

t5403: introduce check_post_checkout helper function

The test file repeatedly uses the same four-line pattern to validate
post-checkout hook arguments: read the args file, then test each of
the three values individually.

Introduce a check_post_checkout helper function that encapsulates this
pattern. This patch does not change test behavior; it prepares the
code for improvement in the next step.

Additionally, the 'post-checkout hook is triggered by clone' test is
improved to validate the hook arguments (old ref, new ref, and flag)
rather than just checking that the hook file was created.

Signed-off-by: Deveshi Dwivedi <deveshigurgaon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>