Testsuite: mark T4945 as expect_broken (#4945)

In commit 7d519dab, the file
T4945.stdout was added to the repository, to make T4945 pass
validatation presumably.

When that test produces output however, there is a bug somewhere, and we
shouldn't hide it. There is a comment in the Makefile which says:

    "When SpecConstr works there are no STUArrays at all"

So here we remove T4945.stdout again, mark T4945 as expect_broken, and
reopen the ticket.

Differential Revision: https://phabricator.haskell.org/D976

Conflicts:
	testsuite/tests/simplCore/should_compile/T4945.stdout
	testsuite/tests/simplCore/should_compile/all.T