tests: Use (ice-9 threads).

'call-with-new-thread' as a core binding is deprecated in 2.2 and
removed in 3.0.

* tests/publish.scm: Use (ice-9 threads).

tests/publish.scm

@@ -45,6 +45,7 @@ (define-module (test-publish)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-64)
+  #:use-module (ice-9 threads)
   #:use-module (ice-9 format)
   #:use-module (ice-9 match)
   #:use-module (ice-9 rdelim))