vimdiff: integrate layout tests in the unit tests framework ('t' folder)

Create a new test case file for the different available merge tools.
Right now it only tests the 'mergetool.vimdiff.layout' option. Other
merge tools might be interested in adding their own tests here too.

Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7609-mergetool--lib.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+test_description='git mergetool
+
+Testing basic merge tools options'
+
+. ./test-lib.sh
+
+test_expect_success 'mergetool --tool=vimdiff creates the expected layout' '
+	. $GIT_BUILD_DIR/mergetools/vimdiff &&
+	run_unit_tests
+'
+
+test_done