Bump up tolerance in one test in tests/misc/test_diffmat.m.

This is needed to get this test to pass on my machine with the new
symmetry-enforcing changes in place.  This gets everything working on my
machine.

Unfortunately, a different test, one which checks for preservation of symmetry,
fails on HM's machine (see issue #1169), so there's still more work to do...
Split results of diffmat symmetry tests into separate pass vector entries.

This makes it possible to tell exactly which of these tests failed without
having to step through line by line or modify the file to print the results of
each.
Fix diffmat symmetry along anti-diagonal.

The mask used to select elements for "symmetrization" misses the anti-diagonal.
We fixed this by handling the anti-diagonal elements separately.  We've done
this in the most direct way possible, it should be possible to come up with a
method less wasteful of memory.
Merge pull request #1172 from chebfun/fix-test-diffmat-2

fix-test-diffmat-2