GitLab CI: avoid confusing error message

Without this package installed, the GitLab CI "test-iuk" test succeeds
but prints this:

    Checking prerequisites...
      test_requires:
        !  Test::LWP::UserAgent is not installed

    ERRORS/WARNINGS FOUND IN PREREQUISITES.  You may wish to install the versions
    of the modules indicated above before proceeding with this installation

    Run 'Build installdeps' to install missing prerequisites.

It does not matter because Test::LWP::UserAgent is only used for tests
that are skipped on GitLab CI, but it did confuse at least one of us:
tails/tails!640 (comment 178808)