test: two builds of one module are refused before anything compiles

The fixture is P -> M and P -> A -> M, where M's own flag --wide
changes a struct it exports. Loaded with --wide --split, P gives M its
args without --wide while A passes them on whole: Build(P) has to fail,
name the module and both paths, and leave no bin/ anywhere. Loaded with
--wide alone, both paths give M the same list, there is one M, and the
build has to go through. Before the fix the first case built cleanly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/DuplicateModule/fixture/m/interfaces/M.cppm

@@ -0,0 +1,12 @@
+export module M;
+import std;
+// --wide gives S more members, so the two builds of M the test asks for
+// differ in what they declare.
+#ifdef WIDE
+export struct S { std::int32_t a; std::int32_t b; std::int64_t pad[4]; };
+#else
+export struct S { std::int32_t a; std::int32_t b; };
+#endif
+export std::int32_t Size() {
+    return static_cast<std::int32_t>(sizeof(S));
+}