Many tests that check the behaviour of symbolic links stored in the
index or the tree objects do not have to be skipped on a filesystem
that lack symbolic link support.

* js/test-ln-s-add:
  t4011: remove SYMLINKS prerequisite
  t6035: use test_ln_s_add to remove SYMLINKS prerequisite
  t3509, t4023, t4114: use test_ln_s_add to remove SYMLINKS prerequisite
  t3100: use test_ln_s_add to remove SYMLINKS prerequisite
  t3030: use test_ln_s_add to remove SYMLINKS prerequisite
  t0000: use test_ln_s_add to remove SYMLINKS prerequisite
  tests: use test_ln_s_add to remove SYMLINKS prerequisite (trivial cases)
  tests: introduce test_ln_s_add
  t3010: modernize style
  test-chmtime: Fix exit code on Windows