compiler: allow emitting tests to an object file

This is fairly straightforward; the actual compiler changes are limited
to the CLI, since `Compilation` already supports this combination.

A new `std.Build` API is introduced to allow representing this. By
passing the `emit_object` option to `std.Build.addTest`, you get a
`Step.Compile` which emits an object file; you can then use that as you
would any other object, such as either installing it for external use,
or linking it into another step.

A standalone test is added to cover the build system API. It builds a
test into an object, and links it into a final executable, which it then
runs.

Using this build system mechanism prevents the build system from
noticing that you're running a `zig test`, so the build runner and test
runner do not communicate over stdio. However, that's okay, because the
real-world use cases for this feature don't want to do that anyway!

Resolves: #23374

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