Use prerequisite tags to skip tests that depend on symbolic links

Many tests depend on that symbolic links work.  This introduces a check
that sets the prerequisite tag SYMLINKS if the file system supports
symbolic links.  Since so many tests have to check for this prerequisite,
we do the check in test-lib.sh, so that we don't need to repeat the test
in many scripts.

To check for 'ln -s' failures, you can use a FAT partition on Linux:

$ mkdosfs -C git-on-fat 1000000
$ sudo mount -o loop,uid=j6t,gid=users,shortname=winnt git-on-fat /mnt

Clone git to /mnt and

$ GIT_SKIP_TESTS='t0001.1[34] t0010 t1301 t403[34] t4129.[47] t5701.7
          t7701.3 t9100 t9101.26 t9119 t9124.[67] t9200.10 t9600.6' \
        make test

(These additionally skipped tests depend on POSIX permissions that FAT on
Linux does not provide.)

Signed-off-by: Johannes Sixt <j6t@kdbg.org>

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