tests/sitecustomize.py

run-tests: move diff generation into TestResult

TestResult is the thing that captures all our test results. It's logical
for diff viewing to be handled there and not inside Test.

While writing this patch, it was discovered that Test.fail() was
printing redundant test result output. This has been removed.

Arguments controlling diffs have been removed from Test.__init__.

try:
    import coverage
    getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
    pass