astgen: enable corpus test for test_all.zig

test_all.zig is 5 lines of @import statements and already produces
matching ZIR. Enable it as a standalone corpus test while keeping
the full corpus test skipped.

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

astgen_test.zig

@@ -848,6 +848,11 @@ fn corpusCheck(gpa: Allocator, name: []const u8, source: [:0]const u8) !void {
     }
 }
 
+test "astgen: corpus test_all.zig" {
+    const gpa = std.testing.allocator;
+    try corpusCheck(gpa, "test_all.zig", @embedFile("test_all.zig"));
+}
+
 test "astgen: corpus" {
     if (true) return error.SkipZigTest;
     const gpa = std.testing.allocator;