test: Add some tests for kconfig.h

The macros in this file are a little confusing and we currently have no
tests to check that they work as expected.

Add some tests which check the macros in C code. Add a few tests which
check that the build errors are generated correctly too, using buildman's
-a option.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/Kconfig b/test/Kconfig
index 9b283a5..a6b463e 100644
--- a/test/Kconfig
+++ b/test/Kconfig
@@ -20,7 +20,7 @@
 config UT_LIB
 	bool "Unit tests for library functions"
 	depends on UNIT_TEST
-	default y
+	default y if !SANDBOX_VPL
 	help
 	  Enables the 'ut lib' command which tests library functions like
 	  memcat(), memcyp(), memmove() and ASN1 compiler/decoder.
@@ -99,5 +99,6 @@
 
 source "test/dm/Kconfig"
 source "test/env/Kconfig"
+source "test/lib/Kconfig"
 source "test/optee/Kconfig"
 source "test/overlay/Kconfig"