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/Step/Compile.zig

@@ -293,6 +293,7 @@ pub const Kind = enum {
     lib,
     obj,
     @"test",
+    test_obj,
 };
 
 pub const HeaderInstallation = union(enum) {
@@ -370,7 +371,7 @@ pub fn create(owner: *std.Build, options: Options) *Compile {
     }
 
     // Avoid the common case of the step name looking like "zig test test".
-    const name_adjusted = if (options.kind == .@"test" and mem.eql(u8, name, "test"))
+    const name_adjusted = if ((options.kind == .@"test" or options.kind == .test_obj) and mem.eql(u8, name, "test"))
         ""
     else
         owner.fmt("{s} ", .{name});
@@ -385,6 +386,7 @@ pub fn create(owner: *std.Build, options: Options) *Compile {
             .lib => "zig build-lib",
             .obj => "zig build-obj",
             .@"test" => "zig test",
+            .test_obj => "zig test-obj",
         },
         name_adjusted,
         @tagName(options.root_module.optimize orelse .Debug),
@@ -396,7 +398,7 @@ pub fn create(owner: *std.Build, options: Options) *Compile {
         .target = target,
         .output_mode = switch (options.kind) {
             .lib => .Lib,
-            .obj => .Obj,
+            .obj, .test_obj => .Obj,
             .exe, .@"test" => .Exe,
         },
         .link_mode = options.linkage,
@@ -1053,6 +1055,7 @@ fn getZigArgs(compile: *Compile, fuzz: bool) ![][]const u8 {
         .exe => "build-exe",
         .obj => "build-obj",
         .@"test" => "test",
+        .test_obj => "test-obj",
     };
     try zig_args.append(cmd);
 
@@ -1222,9 +1225,9 @@ fn getZigArgs(compile: *Compile, fuzz: bool) ![][]const u8 {
                             switch (other.kind) {
                                 .exe => return step.fail("cannot link with an executable build artifact", .{}),
                                 .@"test" => return step.fail("cannot link with a test", .{}),
-                                .obj => {
+                                .obj, .test_obj => {
                                     const included_in_lib_or_obj = !my_responsibility and
-                                        (dep_compile.kind == .lib or dep_compile.kind == .obj);
+                                        (dep_compile.kind == .lib or dep_compile.kind == .obj or dep_compile.kind == .test_obj);
                                     if (!already_linked and !included_in_lib_or_obj) {
                                         try zig_args.append(other.getEmittedBin().getPath2(b, step));
                                         total_linker_objects += 1;

lib/std/Build/Step/InstallArtifact.zig

@@ -56,7 +56,7 @@ pub fn create(owner: *std.Build, artifact: *Step.Compile, options: Options) *Ins
     const dest_dir: ?InstallDir = switch (options.dest_dir) {
         .disabled => null,
         .default => switch (artifact.kind) {
-            .obj => @panic("object files have no standard installation procedure"),
+            .obj, .test_obj => @panic("object files have no standard installation procedure"),
             .exe, .@"test" => .bin,
             .lib => if (artifact.isDll()) .bin else .lib,
         },