tests: Remove duplicate field initializers.

tests: Remove duplicate field initializers.

Fixes a regression introduced in
c2dcff41c2e47f5f978f467864d5ed7829939884, whereby many tests in
'tests/packages.scm' would trigger a syntax error due to duplicate field
intializers in forms like:

  (dummy-package "foo" (version "0"))

* guix/tests.scm (dummy-package, dummy-origin): Rewrite to inherit from
a base record.  This restores the semantics from before
c2dcff41c2e47f5f978f467864d5ed7829939884.
* tests/services.scm ("instantiate-missing-services, indirect"): Remove
duplicate 'extensions' field.