tests: Use variable redirections to dup std{out,err}

Previously, we hardcoded file descriptors 3 and 4 for duplicating
stdandard output/error respectively.  In preparation for keeping
inherited FDs open, switch to using bash's variable redirection feature
to dynamically assign FDs.

This feature is only available from bash 4.1 onwards, so this marks the
end of our support for bash 3.  macOS users will need to install a
modern bash version to run our tests.

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