tests: Add system installation test.

* gnu/tests.scm (define-os-with-source): New macro.
* gnu/tests/install.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* build-aux/run-system-tests.scm (%system-tests): Likewise.

build-aux/run-system-tests.scm

@@ -18,6 +18,7 @@
 
 (define-module (run-system-tests)
   #:use-module (gnu tests base)
+  #:use-module (gnu tests install)
   #:use-module (guix store)
   #:use-module (guix monads)
   #:use-module (guix derivations)
@@ -45,7 +46,8 @@
          (lift1 reverse %store-monad))))
 
 (define %system-tests
-  (list %test-basic-os))
+  (list %test-basic-os
+        %test-installed-os))
 
 (define (run-system-tests . args)
   (with-store store