Fix undefined format-to-string errors

A couple of files were referencing format-to-string without having it
imported and available. This change makes it available.

tests/http-testing.dylan:88: Serious warning - Reference to undefined
binding {format-to-string in http-testing}.
----------------
  test-url(format-to-string("/x?n=%d", n))
----------------

client/tests/http-client-test-suite.dylan:287: Serious warning -
Reference to undefined binding {format-to-string in
http-client-test-suite}.

----------------
  check-equal(format-to-string("chunked request of size %d received correctly",
----------------