Fix checked teardown calls going into infinate loop in CK_NOFORK mode

Fix checked teardown calls going into infinate loop in CK_NOFORK mode

A test can be created with a checked teardown, where the teardown
would call some Check asserts. If the test was run in CK_NOFORK
mode and one of the asserts in the teardown failed, longjmp()
would be called, which would result in the setjmp() being returned
to which monitored the test function, and the checked teardowns
executed again. This lead to an infinate loop.

The fix* for this is to execute the checked teardowns in their
own setjmp() block. A test is added to ensure that CK_NOFORK
with a failed checked teardown does not go into an infinate loop.

In addition, the comments for the checked and unchecked
fixture API are updated to be more clear as to what occurs
for CK_FORK and CK_NOFORK mode.

* This reveals another problem: a failed checked teardown
reacts differently for CK_FORK and CK_NOFORK mode.

CK_FORK: the test is marked as failed
CK_NOFORK: the test is marked as passed

More work will need to be done to allow CK_NOFORK mode to also
cause a test to fail if the checked teardown fails.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@1150 64e312b2-a51f-0410-8e61-82d0ca0eb02a