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__.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..