Reported by Daniel Kochmański <dkochmanski@hellsgate.pl>.
Fixes <http://bugs.gnu.org/19888>.
* guix/tests.scm (shebang-too-long?): New procedure.
* tests/builders.scm ("gnu-build"): Conditionalize on
not (shebang-too-long?).
* tests/packages.scm ("GNU Make, bootstrap"): Likewise.
* tests/guix-package.sh (shebang_not_too_long): New function.
Use it to determine whether to build 'gnu-make-boot0'.