target-xtensa: allow using core configuration in tests

Add path to the core configuration directory to test build command and
replace .include asm directive with #include to enable preprocessing.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
diff --git a/tests/tcg/xtensa/test_extui.S b/tests/tcg/xtensa/test_extui.S
index 5d55451..c32bb82 100644
--- a/tests/tcg/xtensa/test_extui.S
+++ b/tests/tcg/xtensa/test_extui.S
@@ -1,4 +1,4 @@
-.include "macros.inc"
+#include "macros.inc"
 
 test_suite extui