[tests] new test for behavior of __FC_ASSERT_FILE in presence of include

tests/syntax/assert_location.c

+#include "assert_location.h"
+
+void c() {
+  assert("I'm in assert_location.c");
+}

tests/syntax/assert_location.h

+#include "assert.h"
+
+void h() {
+  assert("I'm in assert_location.h");
+}