* guix/tests.scm (network-reachable?): New procedure.
* tests/builders.scm (network-reachable?): Remove.
Replace references to it with calls to the new 'network-reachable?'
procedure.
* tests/derivations.scm (%coreutils): Use 'network-reachable?' instead
of 'getaddrinfo'.
* tests/packages.scm: Likewise.
* tests/union.scm: Likewise.
* guix/tests.scm (%substitute-directory): New variable.
(call-with-derivation-narinfo): Use it.
(call-with-derivation-substitute): New procedure.
(with-derivation-substitute): New macro.
* tests/store.scm ("substitute"): Use 'with-derivation-substitute'.
("substitute, corrupt output hash"): Likewise.
* guix/tests.scm (derivation-narinfo): Turn 'nar' into a keyword
parameter. Add #:sha256 parameter, and honor it.
(call-with-derivation-narinfo): Add #:sha256 and pass it to
'derivation-narinfo'.
(with-derivation-narinfo): Extend with support for (sha256 => value).
* tests/store.scm ("substitute query"): Use 'with-derivation-narinfo'.
("substitute"): Likewise.
("substitute, corrupt output hash"): Likewise.
("substitute --fallback"): Likewise.
* tests/derivations.scm: Remove Emacs local variable.
* guix/tests.scm (derivation-narinfo, call-with-derivation-narinfo): New
procedures.
(with-derivation-narinfo): New macro.
* tests/derivations.scm ("derivation-prerequisites-to-build and
substitutes"): Use them.
