testsuite: Fix cabal01 for real this time

Somehow the previous version passed on master but fails on ghc-8.2.
Will look deeper later.

testsuite/tests/cabal/cabal01/test.cabal

@@ -12,5 +12,5 @@ Main-is: MainA.hs
 Extensions: OverlappingInstances
 
 Executable: testB
-Other-Modules: B.A
+Other-Modules: A, B.A
 Main-is: B/MainB.hs