compiletest: add a new diff for compare-out-by-lines tests.

[Ozzy1423]

Previously, when comparing output by lines, only the actual diff was shown. This is unhelpful since we expect lines to be shuffled around.

With this new print, we can see the exact lines that are missing or have appeared without the noise of the moved around lines.

Example, in this case a note has moved slightly so there is one more separator line "|":

+	   |
+	   = help: maybe it is overwritten before being read?

Compare output by lines enabled, diff by lines:
Expected contains these lines that are not in actual:
(no lines found)
Actual contains these lines that are not in expected:
   |

The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args liveness/liveness-consts.rs`

r? @jieyouxu
cc @zetanumbers @ywxt

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu