test: improve known broken tests support

test: improve known broken tests support

There is existing support for broken tests.  But it is not convenient
to use.  The primary issue is that we have to maintain a set of
test_expect_*_failure functions which are equivalent to the normal
test_expect_* counterparts except for what functions are called for
result reporting.  The patch adds test_subtest_known_broken function
which marks a subset as broken, making the normal test_expect_*
functions behave as test_expect_*_failure.  All test_expect_*_failure
functions are removed.  Test_known_broken_failure_ is changed to
format details the same way as test_failure_ does.

Another benefit of this change is that the diff when a broken test is
fixed would be small and nice.

Documentation is updated accordingly.