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

lib/std/Build/Module.zig

@@ -474,7 +474,7 @@ pub fn addObjectFile(m: *Module, object: LazyPath) void {
 }
 
 pub fn addObject(m: *Module, object: *Step.Compile) void {
-    assert(object.kind == .obj);
+    assert(object.kind == .obj or object.kind == .test_obj);
     m.linkLibraryOrObject(object);
 }