tests: Add lib/b64_decode-test test case

Implement unit tests for lib/b64_decode module.

Signed-off-by: Anna Karas <aka@semihalf.com>
Change-Id: Id5fe9272e30eaff3d086a95241b3819101089c2b
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42313
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

tests/lib/Makefile.inc

@@ -1,6 +1,10 @@
 # SPDX-License-Identifier: GPL-2.0-only
 
 tests-y += string-test
+tests-y += b64_decode-test
 
 string-test-srcs += tests/lib/string-test.c
 string-test-srcs += src/lib/string.c
+
+b64_decode-test-srcs += tests/lib/b64_decode-test.c
+b64_decode-test-srcs += src/lib/b64_decode.c