build: Run all the tests against the just-built daemon.

* test-env.in: New file.
* configure.ac: Add it to `AC_CONFIG_FILES' and `commands-exec'.
* config-daemon.ac: Set and substitute `GUIX_TEST_ROOT'.
* Makefile.am (SCM_LOG_COMPILER, SH_LOG_COMPILER): Use it in lieu of
  `pre-inst-env'.
* daemon.am (test_root): Remove
  (AM_TESTS_ENVIRONMENT): Remove `TEST_ROOT'.
  (clean-local): Use $(GUIX_TEST_ROOT); make files writable before
  removing them.guix_test_root
* tests/guix-daemon.sh: Remove `NIX_' variable settings; don't launch
  `guix-daemon'.

daemon.am

@@ -155,15 +155,12 @@ EXTRA_DIST +=					\
   nix/AUTHORS					\
   nix/COPYING
 
-# Temporary directory used to store the daemon's data.
-test_root = $(abs_top_builddir)/test-tmp
-
 AM_TESTS_ENVIRONMENT +=				\
-  top_builddir="$(abs_top_builddir)"		\
-  TEST_ROOT="$(test_root)"
+  top_builddir="$(abs_top_builddir)"
 
 TESTS +=					\
   tests/guix-daemon.sh
 
 clean-local:
-	-rm -rf "$(test_root)"
+	-find "$(GUIX_TEST_ROOT)" | xargs chmod +w
+	-rm -rf "$(GUIX_TEST_ROOT)"