* 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.