testing: show extra information on failure

[andrewliebenow]

[andrewliebenow]

Old:

---- tr::tr_octal_above_one_byte_value stdout ----
thread 'tr::tr_octal_above_one_byte_value' panicked at plib/src/testing.rs:85:5:
assertion `left == right` failed
  left: "(Ł)"
 right: "Ł)"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

New:

---- tr::tr_octal_above_one_byte_value stdout ----
thread 'tr::tr_octal_above_one_byte_value' panicked at plib/src/testing.rs:229:9:
------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------
Test failed with 2 total failure types
--------------------------------------------------------------------------------

Command executed
--------------------------------------------------------------------------------
tr -d \501
--------------------------------------------------------------------------------

Failure 1 of 2: stdout differs from what was expected
--------------------------------------------------------------------------------

Actual
--------------------------------------------------------------------------------
(Ł)
--------------------------------------------------------------------------------

Expected
--------------------------------------------------------------------------------
Ł)
--------------------------------------------------------------------------------

Failure 2 of 2: stderr differs from what was expected
--------------------------------------------------------------------------------

Actual
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

Expected
--------------------------------------------------------------------------------
tr: warning: the ambiguous octal escape \501 is being interpreted as the 2-byte sequence \050, 1

--------------------------------------------------------------------------------


stderr, for diagnosing failure
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------------------------------

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[andrewliebenow]

The new format might look a bit verbose, but I've repeatedly run up against:

1. Not remembering what left/right correspond to (actual/expected)
2. Not knowing if the comparison is in stdout or stderr
3. Not having access to any additional information beyond that which is provided by the first failed assertion