fix(libtest): Deprecate '--logfile'

[epage]

rust-lang/testing-devex-team#9 proposed changing the behavior of --logfile. The given reasons were:

(1) Bazel can't programmatically process stdout. This seems like a limitation in Bazel and we recommend focusing on that. If we look at the wider Rust ecosystem, Rustc and Cargo don't support any such mechanism and the Cargo team rejected having one. Expecting this in libtest when its not supported elsewhere seems too specialized.

(2) Tests that leak out non-programmatic output that intermixes with programmatic output. We acknowledge this is a problem to be evaluated but we need to make sure we are stepping back and gathering requirements, rather than assuming --logfile will fit the needs.

Independent of the motive, regarding using or changing --logfile

(1) Most ways to do it would be a breaking change, like if we respect any stable --format. As suggested above, we could specialize this to new --format values but that would be confusing for some values to apply but not others.

(2) Other ways of solving this add new features to libtest when we are instead wanting to limit the feature set it has to minimize the compatibility surface that has to be maintained and the burden it would put on third party harnesses which are a focus area. Examples include --format compact or a --log-format flag

(3) The existence of --logfile dates back quite a ways (5cc050b, #2127) and the history gives the
impression this more of slipped through rather than being an intended feature (see also
#82350 (comment)). Deprecation would better match to how it has been treated. By deprecating this, we do not expect custom test harnesses (rust-lang/testing-devex-team#2) to implement this.

T-testing-devex held an FCP for deprecating in rust-lang/testing-devex-team#9 though according to
RFC #3455, this is still subject to final approval from T-libs-api.

Closes rust-lang/testing-devex-team#9