meson: introduce kwargs variable for tests

meson: introduce kwargs variable for tests

Meson has the ability to create a kwargs dictionary that can then be
passed to any function call with the `kwargs:` positional argument. This
allows one to deduplicate common parameters that one wishes to pass to
several different function invocations.

Our tests already have one common parameter that we use everywhere,
"timeout", and we're about to add a second common parameter in the next
commit. Let's prepare for this by introducing `test_kwargs` so that we
can deduplicate these common arguments.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>