t3030: use test_ln_s_add to remove SYMLINKS prerequisite

t3030: use test_ln_s_add to remove SYMLINKS prerequisite

The test cases include many corner-cases of merge-recursive's behavior,
some of them involve type changes and symbolic links. All cases, including
those that are protected by SYMLINKS check only whether the result of
merge-recursive is correctly stored in the database and the index; the
file system is not investigated. Use test_ln_s_add to enter a symbolic
link in the index in the test setup and run the tests without the
SYMLINKS prerequisite.

Notice that one test that has the SYMLINKS protection removed is an
expect_failure. There is a possibility that the test fails differently
depending on whether SYMLINKS is present or not; but this is not the case
presently.

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