tests: use test_ln_s_add to remove SYMLINKS prerequisite (trivial cases)

There are many instances where the treatment of symbolic links in the
object model and the algorithms are tested, but where it is not
necessary to actually have a symbolic link in the worktree. Make
adjustments to the tests and remove the SYMLINKS prerequisite when
appropriate in trivial cases, where "trivial" means:

- merely a replacement of 'ln -s a b && git add b' by test_ln_s_add
  is needed;

- a test for symbolic link on the file system can be split off (and
  remains protected by SYMLINKS);

- existing code is equivalent to test_ln_s_add.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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