Show expected vs. actual output on test failure

Show expected vs. actual output on test failure

This patch changes 'make check' output to show sparse output compared to
expected results upon unexpected test failure. For example,
static-forward-decl.c output would look like this if it would not be tagged as
"known to fail":

       TEST     static forward declaration (static-forward-decl.c)
  error: actual error text does not match expected error text.
  --- static-forward-decl.c.error.expected	2011-08-22 06:29:40.000000000 +0000
  +++ static-forward-decl.c.error.got	2011-08-22 06:29:40.000000000 +0000
  @@ -0,0 +1 @@
  +static-forward-decl.c:3:5: warning: symbol 'f' was not declared. Should it be static?
  error: see static-forward-decl.c.error.* for further investigation.
  info: test 'static-forward-decl.c' is known to fail

This makes it easier to detect and analyze test breakage.

Cc: Christopher Li <sparse@chrisli.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Christopher Li <sparse@chrisli.org>