Test the PSA alternative header configuration macros

Test that MBEDTLS_PSA_CRYPTO_PLATFORM_FILE and
MBEDTLS_PSA_CRYPTO_STRUCT_FILE can be set to files in a directory that comes
after the standard directory in the include file search path.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/include/alt-extra/psa/crypto.h b/tests/include/alt-extra/psa/crypto.h
new file mode 100644
index 0000000..005f3ae
--- /dev/null
+++ b/tests/include/alt-extra/psa/crypto.h
@@ -0,0 +1,7 @@
+/* The goal of the include/alt-extra directory is to test what happens
+ * if certain files come _after_ the normal include directory.
+ * Make sure that if the alt-extra directory comes before the normal
+ * directory (so we wouldn't be achieving our test objective), the build
+ * will fail.
+ */
+#error "The normal include directory must come first in the include path"