tests: Skip network-dependent tests when the network is unreachable.

* tests/builders.scm (network-reachable?): New variable.
  ("url-fetch", "gnu-build"): Skip unless NETWORK-REACHABLE?.
* tests/derivations.scm (%coreutils): Check for network access.
  ("build-expression->derivation with one input"): Skip when %COREUTILS
  is #f.
* tests/guix-package.sh: Skip installation of GNU Make when the network
  is unreachable.
* tests/packages.scm ("GNU Make, bootstrap"): Likewise.
* tests/union.scm ("union-build"): Likewise.