tests: Add tests for "guix pack".

tests: Add tests for "guix pack".

* guix/scripts/pack.scm (bootstrap-xz): New variable.
  (%options) <--bootstrap>: New option.
  (show-help): Document the new --bootstrap option.
  (guix-pack): When --bootstrap is specified, use the bootstrap Guile,
  tar, and xz to build the pack, and do not use any profile hooks or
  locales.
* doc/guix.texi (Invoking guix pull): Document the new --bootstrap
  option.
* tests/guix-pack.sh: New file.
* Makefile.am (SH_TESTS): Add guix-pack.sh.
* gnu/packages/package-management.scm (guix) <inputs>: Add util-linux.