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/a/interfaces/A.cppm

@@ -0,0 +1,6 @@
+export module A;
+import std;
+import M;
+export std::int32_t ASize() {
+    return Size();
+}