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>

stage0_test_root.zig

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