store stdout/stderr output from tests as bytes

[tdyas]

Problem

Pants expects test output to be in UTF-8 even though this assumption is not a valid to make given that test output comes from process invocations. As described in #19121, this causes a UnicodeDecodeError under the conditions mentioned there.

Solution

Instead of storing a str, just store the binary output for stdout and stderr from the underlying Process to avoid string encoding issues as much as possible. When creating a message for output, only then decode the stings as UTF-8 and replace invalid characters.

The stdout and stderr attributes are renamed to stdout_bytes and stderr_bytes, respectively, Introduce stdout and stderr properties so that external plugins get a deprecation warning.

Fixes #19121.

[stuhood]

Thanks!

[tdyas]

Rebased and added a test to ensure invalid UTF-8 still allows .message to render output.

[tdyas]

@benjyw: Thoughts on the interface change for TestResult? E.g., stdout -> stdout_bytes.

[benjyw]

I think that's an excellent API change. Given that we ourselves got confused by this, being explicit in the name is good.