Remove XFAIL_TESTS from Makefile.am

The variable XFAIL_TESTS is not used anymore since commit e90073f7ddc.

The description for the known failures are moved into the respective tests as
comments.

The following descriptions were dropped:

- surface-pattern-big: Didn't really explain the failure
- big-line: Test isn't failing any more
- self-intersecting: Only XFAIL on quartz, but description doesn't match this

The following tests don't have a xfail reference image and seem to fail just
because of not having a reference image at all (I kept their description for
now):

big-trap, long-lines, self-copy-overlap

Signed-off-by: Uli Schlachter <psychon@znc.in>
README: Don't mention XFAIL_TESTS anymore

The variable XFAIL_TESTS is not used anymore since commit e90073f7ddc.

Instead, we now have special reference images that show the wrong output and the
test suite fails a test if it does not match the expected, wrong output.

Signed-off-by: Uli Schlachter <psychon@znc.in>