tests: move copy/pasted PERL + Test::More checks to a lib-perl.sh

tests: move copy/pasted PERL + Test::More checks to a lib-perl.sh

Since the original "perl -MTest::More" prerequisite check was added in
[1] it's been copy/pasted in [2], [3] and [4]. As we'll be changing
these codepaths in a subsequent commit let's consolidate these.

While we're at it let's move these to a lazy prereq, and make them
conform to our usual coding style (e.g. "\nthen", not "; then").

1. e46f9c8161a (t9700: skip when Test::More is not available,
   2008-06-29)
2. 5e9637c6297 (i18n: add infrastructure for translating Git with
   gettext, 2011-11-18)
3. 8d314d7afec (send-email: reduce dependencies impact on
   parse_address_line, 2015-07-07)
4. f07eeed123b (git-credential-netrc: adapt to test framework for git,
   2018-05-12)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>