Add freopen special-case tests: thread cancellation

Add tests of freopen adding or removing "c" (non-cancelling I/O) from
the mode string (so completing my planned tests of freopen with
different features used in the mode strings).  Note that it's in the
nature of the uncertain time at which cancellation might act (possibly
during freopen, possibly during subsequent reads) that these can leak
memory or file descriptors, so these do not include leak tests.

Tested for x86_64.

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