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__.
$ hg init
$ python -c 'file("a", "wb").write("confuse str.splitlines\nembedded\rnewline\n")'
$ hg ci -Ama -d '1 0'
adding a
$ echo clean diff >> a
$ hg ci -mb -d '2 0'
$ hg diff -r0 -r1
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a Thu Jan 01 00:00:01 1970 +0000
+++ b/a Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@
confuse str.splitlines
embedded\r (no-eol) (esc)
newline
+clean diff