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. e46f9c8161 (t9700: skip when Test::More is not available,
   2008-06-29)
2. 5e9637c629 (i18n: add infrastructure for translating Git with
   gettext, 2011-11-18)
3. 8d314d7afe (send-email: reduce dependencies impact on
   parse_address_line, 2015-07-07)
4. f07eeed123 (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>

contrib/credential/netrc/t-git-credential-netrc.sh

@@ -3,16 +3,9 @@
 	cd ../../../t
 	test_description='git-credential-netrc'
 	. ./test-lib.sh
+	. "$TEST_DIRECTORY"/lib-perl.sh
 
-	if ! test_have_prereq PERL; then
-		skip_all='skipping perl interface tests, perl not available'
-		test_done
-	fi
-
-	perl -MTest::More -e 0 2>/dev/null || {
-		skip_all="Perl Test::More unavailable, skipping test"
-		test_done
-	}
+	skip_all_if_no_Test_More
 
 	# set up test repository