Update test to reflect positive outcome for -fno-fortran-main

Author: mjklemm

flang/test/Driver/no-duplicate-main.f90

@@ -4,6 +4,8 @@
 ! RUN: %flang -o %t -c %s
 ! RUN: not %flang -o %t.exe %t %t.c-object 2>&1
 
+! RUN: %flang -fno-fortran-main -o %t.exe %t %t.c-object 2>&1
+
 ! TODO: potentially add further checks to ensure that proper
 !       linker error messages are detected and checked via
 !       FileCheck.