tests: Add lib/string-test test case

Show a basic example of how unit testing can be applied for the coreboot
project. Add a test harness for lib/string.c module.

TEST=Install cmocka via appropriate command:
sudo apt-get install -y libcmocka-dev
sudo emerge dev-util/cmocka
yum install libcmocka-devel
* Build and run unit tests via `make unit-tests`
* Check the output to see that tests passed.

Signed-off-by: Jan Dabros <jsd@semihalf.com>
Change-Id: Ibf5554d1e99a393721a66bdd35af0122c2e412c4
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40538
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>

tests/lib/Makefile.inc

@@ -0,0 +1,20 @@
+##
+## This file is part of the coreboot project.
+##
+## This program is free software; you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation; version 2 of the License.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+
+# object filest should be under build/tests/ build/test/src/ build/test/run/
+# two examples - first should be simply string.c, second should use -wrap
+
+tests-y += string-test
+
+string-test-srcs += tests/lib/string-test.c
+string-test-srcs += src/lib/string.c