Revert "Fix edge case test failure in ext/POSIX/t/mb.t"

Revert "Fix edge case test failure in ext/POSIX/t/mb.t"

This reverts commit 69b89a0f0bb2cbb4c1607e78c3b414bf45244bea.

This commit generated two test failures on Linux when perl was built
with config_args like these:

'-des -Dusedevel -Dcc=clang -Accflags=-Werror=declaration-after-statement -g -fno-omit-frame-pointer -fsanitize=address  -fno-common -fsanitize-blacklist=/home/jkeenan/gitwork/perl/asan_ignore -Aldflags=-fsanitize=address'

See: https://rt.perl.org/Ticket/Display.html?id=134182#txn-1639258

Reverting from blead; will create branch for testing.

Signed-off-by: James E Keenan <jkeenan@cpan.org>