Rollup merge of #109484 - fortanix:raoul/bugfix_libtest_json_output, r=pietroalbini

Bugfix: avoid panic on invalid json output from libtest

#108659 introduces a custom test display implementation. It does so by using libtest to output json. The stdout is read and parsed; The code trims the line read and checks whether it starts with a `{` and ends with a `}`. If so, it concludes that it must be a json encoded `Message`. Unfortunately, this does not work in all cases:

- This assumes that tests running with `--nocapture` will never start and end lines with `{` and `}` characters
- Output is generated by issuing multiple `write_message` [statements](https://github.com/rust-lang/rust/blob/master/library/test/src/formatters/json.rs#L33-L60). Where only the last one issues a `\n`. This likely results in a race condition as we see multiple json outputs on the same line when running tests for the `x86_64-fortanix-unknown-sgx` target:
```
10:21:04 �[0m�[0m�[1m�[32m     Running�[0m tests/run-time-detect.rs (build/x86_64-unknown-linux-gnu/stage1-std/x86_64-fortanix-unknown-sgx/release/deps/run_time_detect-8c66026bd4b1871a)
10:21:04
10:21:04 running 1 tests
10:21:04 test x86_all ... ok
10:21:04 �[0m�[0m�[1m�[32m     Running�[0m tests/thread.rs (build/x86_64-unknown-linux-gnu/stage1-std/x86_64-fortanix-unknown-sgx/release/deps/thread-ed5456a7d80a6193)
10:21:04 thread 'main' panicked at 'failed to parse libtest json output; error: trailing characters at line 1 column 135, line: "{ \"type\": \"suite\", \"event\": \"ok\", \"passed\": 1, \"failed\": 0, \"ignored\": 0, \"measured\": 0, \"filtered_out\": 0, \"exec_time\": 0.000725911 }{ \"type\": \"suite\", \"event\": \"started\", \"test_count\": 1 }\n"', render_tests.rs:108:25
```

This PR implements a partial fix by being much more conservative of what it asserts is a valid json encoded `Message`. This prevents panics, but still does not resolve the race condition. A discussion is needed where this race condition comes from exactly and how it best can be avoided.

cc: `@jethrogb,` `@pietroalbini`

Author: matthiaskrgr