Tests: SBCL passes test-lisp and test-upgrade
Also:
* big refactoring of test infrastructure
* contrib/debug.lisp and (asdf/driver:asdf-debug) for debugging.
* remove aif and it, use the cleaner if-bind.
* backtrace support, with improvements from trivial-backtrace.
* integration of asdf-condition-control (originally from xcvb-driver)
* simplify the upgrade heuristic: always tries to upgrade once.
  The previous heuristic was too complex with too many failure cases.
  You are thus assumed to want a given version asdf
  if and only if it is in your source-registry.

Remove debugging code from previous commit.

The issue fixed is that most Lisps don't like
*default-pathname-defaults* being a logical-pathname.
Added a test for logical-pathname functionality;
it all works on CCL, and works better on other Lisps after the fix,
but there are still logical-pathname issues on SBCL, CLISP and more.
Also, a package cleanup of our test suite.

Nikodemus reported on the list an infortunate interaction with quicklisp.
such if when you had a system installed both in quicklisp and outside
quicklisp, say flexi-streams, and the .asd defsystem'ed an auxiliary system,
say flexi-streams-test, and you tried to load the version outside quicklisp,
e.g. by putting it first in the *central-registry*, then ASDF would
start loading the non-quicklisp .asd; during parse-component-form for the
auxiliary system, find-system would be called and then locate the quicklisp
version, since it can't be found by file name, but quicklisp includes it
in its database; ASDF would then load the quicklisp .asd; during the
parse-component-form for the main system, it would call find-system,
and the non-quicklisp .asd would be found, and so on in an infinite loop.

This patch refactors defsystem to put all the work in a function
will a trivial macro-expansion. The function pushes the system being
currently defined into a list of such systems that is specially consulted
first by search-for-system-definition, hopefully avoiding any loop.

https://bugs.launchpad.net/asdf/+bug/779935. Grrrrrrr. Add proper test.

Test support for symlinked .asd's to prevent future regressions
as per the 2.014.9 bug fixed in 2.014.16.