Testsuite: pass -i argument to runghc invocations

This commit modifies the generation of runghc commands to pass an
additional -i argument to account for the change in working directory.

This ensures that, in the testsuite, runghc is able to see other modules.
For example, an invocation of runghc to run a Custom Setup script will
now properly see the modules imported, e.g. if one has a directory
structure like

  test.cabal
  Setup.hs
  SetupDep.hs

and Setup.hs imports SetupDep.hs.