tests: Fix random failure when base64 string starts with URI

At random times, the uri test was failing with some error like this:

```
openssl storeutl -text "$uri"
Couldn't open file or uri URI7xnNajqYJnkZTktp42UM2yp38/G8hCR7QWsnuREHRXLaVoqVJouteOg==
40A7BAF0F27F0000:error:16000069:STORE routines:ossl_store_get0_loader_int:unregistered scheme:../crypto/store/store_register.c:237:scheme=file
40A7BAF0F27F0000:error:80000002:system library:file_open:No such file or directory:../providers/implementations/storemgmt/file_store.c:267:calling stat(URI7xnNajqYJnkZTktp42UM2yp38/G8hCR7QWsnuREHRXLaVoqVJouteOg==)
```

Signed-off-by: Jakub Jelen <jjelen@redhat.com>

tests/turi

@@ -14,7 +14,7 @@ if [ $FAIL -ne 0 ]; then
     exit 1
 fi
 
-URISonly=$(echo "$helper_output" | grep "^URI")
+URISonly=$(echo "$helper_output" | grep "^URI pkcs11:")
 # poor mans mapfile for bash 3 on macos
 declare -a URIS
 while read -r var; do