wcc: @test marker attributes + runner

Hare-style `@test fn check_foo() void = { ... }` now parses. The
attribute is recognised by making the args list optional in
parseattrs: `@symbol("rt_syscall")` still requires the parens;
`@test` doesn't. Same change mirrored in lib/ww/parse/decl.ww.

The runner (test/wcc/910_at_test.c) scans a fixture for
`@test fn IDENT(`, synthesises a wrapper `main()` that calls each
test fn, builds it via `ww run`, and asserts exit 0. A failing
@test would either explicitly call abort or trip a runtime trap
(div-by-zero, etc.) and the whole driver exits non-zero.

The 910_at_test target sits alongside the existing C-side test
binaries; `make test` now runs 20 tests instead of 19.

Fixture: test/wcc/data/attest_pass.ww exercises two passing tests
(simple arithmetic and a match-with-yield).

cmd/wcc/parse.c

@@ -1241,9 +1241,13 @@ parseattrs(Parser *p)
 		advance(p);
 		Node *a = newnode(p->a, N_ATTR, pp);
 		a->str = expectident(p);
-		expect(p, TK_LPAREN);
-		a->list = parsearglist(p, TK_RPAREN);
-		expect(p, TK_RPAREN);
+		/* `@name(args...)` for FFI-style attributes;
+		 * `@name` (no parens) for marker-only attributes like
+		 * `@test`. */
+		if (accept(p, TK_LPAREN)) {
+			a->list = parsearglist(p, TK_RPAREN);
+			expect(p, TK_RPAREN);
+		}
 		if (head == NULL) head = a;
 		else tail->next = a;
 		tail = a;