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).

Makefile

@@ -212,6 +212,7 @@ $(BIN) $(LIB) $(OBJ)/wcc $(OBJ)/ww $(OBJ)/wwdump $(OBJ)/w6c $(OBJ)/w6a $(OBJ)/w6
 TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_w6c $(BIN)/test_w6a $(BIN)/test_w6l $(BIN)/test_arch \
 	  $(BIN)/test_e2e $(BIN)/test_ffi $(BIN)/test_dyn $(BIN)/test_stdlib \
+	  $(BIN)/test_at_test \
 	  $(BIN)/test_selfhost $(BIN)/test_w6a_ww $(BIN)/test_w6l_ww \
 	  $(BIN)/test_w6c_ww $(BIN)/test_ww_ww $(BIN)/test_self_rebuild \
 	  $(BIN)/test_dyn_ww
@@ -255,6 +256,10 @@ $(BIN)/test_dyn: test/wcc/810_dyn.c $(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 $(BIN)/test_stdlib: test/wcc/900_stdlib.c $(BIN)/w6c | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_at_test: test/wcc/910_at_test.c $(BIN)/ww $(BIN)/w6c $(BIN)/w6a \
+		$(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_selfhost: test/wcc/990_selfhost.c $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww $(BIN)/wwdump $(BIN)/wwdump_ww $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<