app_update: fix test failure for get_app_elf_sha256 test

Commit fc03161f updated esp_ota_get_app_elf_sha256 to store and return
X number of bytes of the hash, but the test case still expected 64 bytes.

Updated test case to use CONFIG value for expected length.

components/app_update/test/test_app_desc.c

@@ -4,7 +4,7 @@
 
 TEST_CASE("esp_ota_get_app_elf_sha256 test", "[esp_app_desc]")
 {
-    const int sha256_hex_len = 64;
+    const int sha256_hex_len = CONFIG_APP_RETRIEVE_LEN_ELF_SHA;
     char dst[sha256_hex_len + 2];
     const char fill = 0xcc;
     int res;