clash-ffi: Make ffi_interface-tests a proper test-suite

There seems to be no reason this was not a test-suite
to begin with. Making it a test-suite makes it
automatically build and ran for nix builds of
clash-ffi.

Author: rowanG077

clash-ffi/clash-ffi.cabal

@@ -77,8 +77,9 @@ library
     Clash.FFI.VPI.Port.Direction
     Clash.FFI.VPI.Reg
 
-executable ffi-interface-tests
+test-suite ffi-interface-tests
   import: common-options
+  type: exitcode-stdio-1.0
   default-extensions:
     DataKinds
     RankNTypes