Fix test

Author: ldionne

libcxx/test/libcxx/headers_in_modulemap.sh.py

@@ -4,7 +4,7 @@
 sys.path.append(sys.argv[1])
 from libcxx.header_information import all_headers, libcxx_include
 
-with open(libcxx_include / "module.modulemap") as f:
+with open(libcxx_include / "module.modulemap.in") as f:
     modulemap = f.read()
 
 isHeaderMissing = False