Avoid intermediate list for unified_diff results
Revert expected and observed results in diffing

this way, removals and additions show the *errors* in the current output
Have each fixture be reported as its own test case

Allows checking multiple failing fixtures, and better reporting on
tests count.
Remove default/empty visit/depart implementations

better to fail on them, if no-behavior is desirable should extend SparseNodeVisitor instead
Merge pull request #19 from masklinn/fixtures-tester-improvements

Improved tests of fixtures.