Removed $in_subtest as it is no longer required

Before this was required to check if the follow_up had been fired by the
end of a subtest. With this fix,
https://github.com/Test-More/test-more/commit/0850e52eb74e4049bd0cfc36d29b5638b321e73b,
it is no longer necessary.
Removed obsolete checks on $called_by_done_testing
Use Test::Stream::ExitMagic::Context to determine if done_testing ran
Tested DummyTap