gnu: guile-for-guile-emacs: Update to 2.1.2-1.15ca784.

* gnu/packages/guile.scm (guile-for-guile-emacs): Update to 2.1.2-1.15ca784.
[source]: Fetch from Guile upstream repository.
[arguments]: Enable tests; replace 'bootstrap phase; disable only broken
tests.
[native-search-paths]: Inherit from guile-2.2.