xapi-log/test: Package the cram test in xapi-log (#6450)

This should fix the current xs-opam failure with xapi master, see for
example the failure here:
https://github.com/last-genius/xs-opam/tree/master

Fixes: bfea6f3 ("CA-409628: Add backtrace logging test")

Author: last-genius

ocaml/libs/log/test/dune

@@ -3,4 +3,5 @@
  (libraries log xapi-stdext-threads threads.posix xapi-backtrace))
 
 (cram
+ (package xapi-log)
  (deps log_test.exe))