Stabilize flags for doctest cross compilation
This makes the following changes in preparation for supporting doctest cross-compiling in cargo:
- Renames `--runtool` and `--runtool-arg` to `--test-runtool` and `--test-runtool-arg` to maintain consistency with other `--test-*` arguments.
- Stabilizes the `--test-runtool` and `--test-runtool-arg`. These are needed in order to support cargo's `target.runner` option which specifies a runner to execute a cross-compiled doctest (for example, qemu).
- Stabilizes the `--enable-per-target-ignores` flag by removing it and making it unconditionally enabled. This makes it possible to disable a doctest on a per-target basis, which I think will be helpful for rolling out this feature.
These changes were suggested in https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/stabilizing.20doctest.20xcompile/near/409281127
The intent is to stabilize the doctest-xcompile feature in cargo. This will help ensure that for projects that do cross-compile testing that their doctests are also covered. Currently there is a somewhat surprising behavior that they are ignored.
Closes https://github.com/rust-lang/rust/issues/64245
try-job: x86_64-msvc-1
610ed826e3