message: Prep for asser_eq failure msg format change.
diff: @@ -215,10 +215,11 @@ test test_hello ... FAILED
  failures:
  ---- test_hello stdout ----
  -<tab>thread 'test_hello' panicked at 'assertion failed: \
  - `(left == right)` (left: \
  - `\"hello\"`, right: `\"nope\"`)', src[/]foo.rs:12
  -")
  +<tab>thread 'test_hello' panicked at 'assertion failed:[..]")
  + .with_stdout_contains("[..]`(left == right)`[..]")
  + .with_stdout_contains("[..]left: `\"hello\"`,[..]")
  + .with_stdout_contains("[..]right: `\"nope\"`[..]")
  + .with_stdout_contains("[..]src[/]foo.rs:12[..]")
  .with_stdout_contains("\
  failures:
  test_hello
  