gnu: ruby-activemodel: Update to 7.0.4.3 and enable tests.

* gnu/packages/rails.scm (ruby-activemodel): Update to 7.0.4.3.  Fix
indentation.
[source]: Use ruby-rails-monorepo.
[arguments]: Remove #:tests? argument.  Add #:phases argument.
[native-inputs]: New field.

gnu/packages/rails.scm

@@ -261,20 +261,22 @@ (define-public ruby-autoprefixer-rails
 (define-public ruby-activemodel
   (package
     (name "ruby-activemodel")
-   (version "6.1.3")
+    (version %ruby-rails-version)
-   (source
+    (source ruby-rails-monorepo)
-    (origin
-     (method url-fetch)
-     (uri (rubygems-uri "activemodel" version))
-     (sha256
-      (base32
-       "07m85r00cd1dzxg65zr9wjrdqppw51b5ka9c5mrz92vnw18kfb70"))))
     (build-system ruby-build-system)
     (arguments
-    '(;; No included tests
+     (list
-      #:tests? #f))
+      #:phases
-   (propagated-inputs
+      #~(modify-phases %standard-phases
-    (list ruby-activesupport))
+          (add-after 'extract-gemspec 'chdir
+            (lambda _
+              (chdir "activemodel")))
+          (add-after 'chdir 'delete-problematic-tests
+            (lambda _
+              ;; We do not want to depend on ruby-railties at this stage.
+              (delete-file "test/cases/railtie_test.rb"))))))
+    (native-inputs (list ruby-bcrypt))
+    (propagated-inputs (list ruby-activesupport))
     (synopsis "Toolkit for building modeling frameworks like Active Record")
     (description
      "This package provides a toolkit for building modeling frameworks like