t/test-terminal: avoid non-inclusive language

t/test-terminal: avoid non-inclusive language

In the ongoing effort to make the Git project a more inclusive place,
let's try to avoid names like "master" where possible.

In this instance, the use of the term `slave` is unfortunately enshrined
in IO::Pty's API. We simply cannot avoid using that word here. But at
least we can get rid of the usage of the word `master` and hope that
IO::Pty will be eventually adjusted, too.

Guessing that IO::Pty might follow Python's lead, we replace the name
`master` by `parent` (hoping that IO::Pty will adopt the parent/child
nomenclature, too).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>