tests: Make 'lit' the default test tool. You can still use 'make check-dg' to

tests: Make 'lit' the default test tool. You can still use 'make check-dg' to
run the tests using DejaGNU, but not for much longer. This is a last call for
DejaGNU supporters, if no one complains soon the DejaGNU support is going to
die.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109997 91177308-0d34-0410-b5e6-96231b3b80d8