Issue 705836: Fix struct.pack(">f", 1e40) to behave consistently

Issue 705836: Fix struct.pack(">f", 1e40) to behave consistently
across platforms:  it should now raise OverflowError on all
platforms.  (Previously it raised OverflowError only on
non IEEE 754 platforms.)

Also fix the (already existing) test for this behaviour
so that it actually raises TestFailed instead of just
referencing it.