tests: Run 'tests/offload.scm' only when BUILD_DAEMON_OFFLOAD is true.

Fixes:

  guix build -e '(@ (gnu packages package-management) guix-minimal)'

* Makefile.am (SCM_TESTS): Move 'tests/offload.scm' within "if
BUILD_DAEMON_OFFLOAD" conditional.

Makefile.am

@@ -462,7 +462,6 @@ SCM_TESTS =					\
   tests/monads.scm				\
   tests/nar.scm				\
   tests/networking.scm				\
-  tests/offload.scm				\
   tests/opam.scm				\
   tests/openpgp.scm				\
   tests/packages.scm				\
@@ -499,6 +498,12 @@ SCM_TESTS =					\
   tests/uuid.scm				\
   tests/workers.scm
 
+if BUILD_DAEMON_OFFLOAD
+SCM_TESTS  += tests/offload.scm
+else
+EXTRA_DIST += tests/offload.scm
+endif
+
 SH_TESTS =					\
   tests/guix-build.sh				\
   tests/guix-build-branch.sh			\