Add Wisun Fan device extended key usage

Add the Wisun extended key usage oid and tests.
diff --git a/tests/suites/test_suite_oid.function b/tests/suites/test_suite_oid.function
index e95e48d..59e700e 100644
--- a/tests/suites/test_suite_oid.function
+++ b/tests/suites/test_suite_oid.function
@@ -32,3 +32,26 @@
     }
 }
 /* END_CASE */
+
+/* BEGIN_CASE */
+void oid_get_extended_key_usage( data_t *oid, char *result_str )
+{
+    mbedtls_asn1_buf asn1_buf = { 0, 0, NULL };
+    int ret;
+    const char *desc;
+
+    asn1_buf.tag = MBEDTLS_ASN1_OID;
+    asn1_buf.p = oid->x;
+    asn1_buf.len = oid->len;
+
+    ret = mbedtls_oid_get_extended_key_usage( &asn1_buf, &desc );
+    if( strlen( result_str ) == 0 )
+    {
+        TEST_ASSERT( ret == MBEDTLS_ERR_OID_NOT_FOUND );
+    }
+    else
+    {
+        TEST_ASSERT( strcmp( ( char * )desc, result_str ) == 0 );
+    }
+}
+/* END_CASE */