When making the `sdist` tarball, we don't really need anything inside
$(TOP)/testsuite in order to do our thing. So make sure we clean it
first to avoid situations like #10406.

With D917 landed, this can actually avoided entirely by fixing the
official release process to instead build an `sdist` //first// from the
clean git repository and then build that (to fixpoint) and test it. Then
the originall clean tarball can be shipped.

But it's nice to be safe in the general case where someone might want to
(in the future) `sdist` out of their build tree.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Reviewed By: thomie

Differential Revision: https://phabricator.haskell.org/D956

GHC Trac Issues: #10406