Fix missed errors in Dynlink C# tests
Fix missed errors in Dynlink C# tests

The C# tests for Dynlink do not check for errors in subcommands.
Simplest fix, done here, is to ensure that the binary files from
previous tests are deleted first to ensure that subsequent tests don't
use them by mistake when their own build commands have failed!

yunxing/ocaml,gerdstolpmann/ocaml,yunxing/ocaml,msprotz/ocaml,msprotz/ocaml,msprotz/ocaml,msprotz/ocaml,yunxing/ocaml,yunxing/ocaml,gerdstolpmann/ocaml,yunxing/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml