Auto merge of #46450 - Gilnaa:libtest_json_output, r=nrc

Libtest json output

A revisit to my [last PR](https://github.com/rust-lang/rust/pull/45923).

Events are now more atomic, printed in a flat hierarchy.

For the normal test output:
```
running 1 test
test f ... FAILED

failures:

---- f stdout ----
	thread 'f' panicked at 'assertion failed: `(left == right)`
  left: `3`,
 right: `4`', f.rs:3:1
note: Run with `RUST_BACKTRACE=1` for a backtrace.

failures:
    f

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
```

The JSON equivalent is:
```
{ "type": "suite", "event": "started", "test_count": "1" }
{ "type": "test", "event": "started", "name": "f" }
{ "type": "test", "event": "failed", "name": "f" }
{ "type": "suite", "event": "failed", "passed": 0, "failed": 1, "allowed_fail": 0, "ignored": 0,  "measured": 0, "filtered_out": "0" }
{ "type": "test_output", "name": "f", "output": "thread 'f' panicked at 'assertion failed: `(left == right)`
  left: `3`,
 right: `4`', f.rs:3:1
note: Run with `RUST_BACKTRACE=1` for a backtrace.
" }
```

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