test: allow prerequisite to be evaluated lazily

test: allow prerequisite to be evaluated lazily

The test prerequisite mechanism is a useful way to allow some tests
in a test script to be skipped in environments that do not support
certain features (e.g. it is pointless to attempt checking how well
symbolic links are handled by Git on filesystems that do not support
them).  It is OK for commonly used prerequisites to be always tested
during start-up of a test script by having a codeblock that tests a
feature and calls test_set_prereq, but for an uncommon feature,
forcing 90% of scripts to pay the same probing overhead for
prerequisite they do not care about is wasteful.

Introduce a mechanism to probe the prerequiste lazily.  Changes are:

 - test_lazy_prereq () function, which takes the name of the
   prerequisite it probes and the script to probe for it, is
   added.  This only registers the name of the prerequiste that can
   be lazily probed and the script to eval (without running).

 - test_have_prereq() function (which is used by test_expect_success
   and also can be called directly by test scripts) learns to look
   at the list of prerequisites that can be lazily probed, and the
   prerequisites that have already been probed that way.  When asked
   for a prerequiste that can be but haven't been probed, the script
   registered with an earlier call to test_lazy_prereq is evaluated
   and the prerequisite is set.

 - test_run_lazy_prereq_() function is a helper to run the probe
   script with the same kind of sandbox as regular tests, helped by
   Jeff King.

Update the codeblock to probe and set SYMLINKS prerequisite using
the new mechanism as an example.

Signed-off-by: Junio C Hamano <gitster@pobox.com>