t5500 & t7403: lose bash-ism "local"

t5500 & t7403: lose bash-ism "local"

In t5500::check_prot_host_port_path(), diagport is not a variable
used elsewhere and the function is not recursively called so this
can simply lose the "local", which may not be supported by shell
(besides, the function liberally clobbers other variables without
making them "local").

t7403::reset_submodule_urls() overrides the "root" variable used
in the test framework for no good reason; its use is not about
temporarily relocating where the test repositories are created.
This assignment can be made not to clobber the variable by moving
them into the subshells it already uses.  Its value is always
$TRASH_DIRECTORY, so we could use it instead there, and this
function that is called only once and its two subshells may not be
necessary (instead, the caller can use "git -C $there config" and
set a value that is derived from $TRASH_DIRECTORY), but this is a
minimum fix that is needed to lose "local".

Helped-by: John Keeping <john@keeping.me.uk>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>