simplify zig0 test setup: single test module, remove zig0_test.zig

Consolidate the two separate test modules (test_mod via
lib/std/zig/zig0_test.zig + astgen_test_mod via stage0_test_root.zig)
into a single test module rooted at stage0_test_root.zig.

The zig0_test.zig bridge approach ran std's parser/tokenizer tests with
C comparison enabled, but the stage0/ test files already do the same
C-vs-Zig comparison directly via @cImport. The only "lost" tests are an
unnamed root test block and a Zig-only fuzz test — no zig0 coverage lost.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build.zig

@@ -1606,40 +1606,19 @@ fn addZig0TestStep(
     valgrind: bool,
     test_timeout: ?[]const u8,
 ) void {
-    const bridge_mod = b.createModule(.{
-        .root_source_file = b.path("stage0/zig0_bridge.zig"),
-        .target = target,
-        .optimize = optimize,
-    });
-    bridge_mod.addIncludePath(b.path("stage0"));
-
-    // Parser + tokenizer tests (hooks into std's test files via zig0 bridge)
     const test_mod = b.createModule(.{
-        .root_source_file = b.path("lib/std/zig/zig0_test.zig"),
-        .optimize = optimize,
-        .target = target,
-    });
-    test_mod.addImport("zig0_bridge", bridge_mod);
-
-    // AstGen tests (standalone C-vs-Zig ZIR comparison)
-    const astgen_test_mod = b.createModule(.{
         .root_source_file = b.path("stage0_test_root.zig"),
         .optimize = optimize,
         .target = target,
     });
-    astgen_test_mod.addIncludePath(b.path("stage0"));
+    test_mod.addIncludePath(b.path("stage0"));
+    addZig0CSources(b, test_mod, cc, optimize);
+    test_mod.linkSystemLibrary("c", .{});
 
     const timeout: ?[]const u8 = test_timeout orelse if (valgrind) null else "10";
 
-    for ([_]struct { mod: *std.Build.Module, name: []const u8 }{
-        .{ .mod = test_mod, .name = "test" },
-        .{ .mod = astgen_test_mod, .name = "astgen_test" },
-    }) |entry| {
-        addZig0CSources(b, entry.mod, cc, optimize);
-        entry.mod.linkSystemLibrary("c", .{});
-
     const test_exe = b.addTest(.{
-            .root_module = entry.mod,
+        .root_module = test_mod,
         .use_llvm = false,
         .use_lld = false,
     });
@@ -1662,10 +1641,9 @@ fn addZig0TestStep(
         step.dependOn(&install.step);
     } else {
         const run = b.addRunArtifact(test_exe);
-            run.step.name = b.fmt("{s} ({s})", .{ entry.name, cc });
+        run.step.name = b.fmt("test ({s})", .{cc});
         step.dependOn(&run.step);
     }
-    }
 }
 
 fn addZig0CSources(

lib/std/zig/zig0_test.zig

@@ -1,6 +0,0 @@
-pub const zig0 = @import("zig0_bridge");
-
-test {
-    _ = @import("parser_test.zig");
-    _ = @import("tokenizer.zig");
-}

stage0_test_root.zig

@@ -1,3 +1,3 @@
 test {
-    _ = @import("stage0/astgen_test.zig");
+    _ = @import("stage0/test_all.zig");
 }