wcc: reject exported/bodyless/duplicate @test shapes

Both frontends, byte-stable fragments; -T-mode checks, so the rows
live in test/package (the fixture compile cell cannot reach -T).

selfhost/cmd/wcc/check.ww

@@ -7134,16 +7134,38 @@ export fn checkfile(c: *checker, file: *syntax.node) void = {
 		let t: *syntax.node = file.list;
 		for (t != nil) {
 			if (t.kind == syntax.nkind.N_FNDECL) {
-				let istest: bool = false;
+				let ntestattr: i32 = 0;
 				let at: *syntax.node = t.attr;
 				for (at != nil) {
 					if (at.kind == syntax.nkind.N_ATTR
 					    && syntax.streq(at.str, "test")) {
-						istest = true;
+						ntestattr += 1;
 					};
 					at = at.next;
 				};
-				if (istest) {
+				// Attribute-shape rejects, fixed order, first
+				// failure wins per fn; wording byte-stable with
+				// the cstage twin (check.c). A silently-dropped
+				// shape here ships a test that never runs.
+				if (ntestattr > 1) {
+					cerr(t.file);
+					cerr(": error: duplicate @test on fn '");
+					cerr(t.str);
+					cerr("'\n");
+					c.errs += 1;
+				} else { if (ntestattr == 1 && t.exported != 0) {
+					cerr(t.file);
+					cerr(": error: @test fn '");
+					cerr(t.str);
+					cerr("' cannot be exported\n");
+					c.errs += 1;
+				} else { if (ntestattr == 1 && t.body == nil) {
+					cerr(t.file);
+					cerr(": error: @test fn '");
+					cerr(t.str);
+					cerr("' needs a body\n");
+					c.errs += 1;
+				} else { if (ntestattr == 1) {
 					// `fn f() void` parses the explicit void
 					// into t.lhs, so void-returning is lhs==nil
 					// OR an N_TNAME "void".
@@ -7172,7 +7194,7 @@ export fn checkfile(c: *checker, file: *syntax.node) void = {
 						rtail = row;
 						ntest += 1i32;
 					};
-				};
+				}; }; }; };
 			};
 			t = t.next;
 		};