skip_if_not() works like stopifnot(), generating
a message automatically based on the first argument.
skip_if_offline() skips tests if an internet connection is not available
using curl::nslookup().
skip_on_cran() skips tests on CRAN, using the NOT_CRAN
environment variable set by devtools.
skip_on_travis() skips tests on Travis CI by inspecting the
TRAVIS environment variable.
skip_on_appveyor() skips tests on AppVeyor by inspecting the
APPVEYOR environment variable.
skip_on_ci() skips tests on continuous integration systems by inspecting
the CI environment variable.
skip_on_covr() skips tests when covr is running by inspecting the
R_COVR environment variable
skip_on_bioc() skips tests on Bioconductor by inspecting the
BBS_HOME environment variable.
skip_if_not_installed() skips a tests if a package is not installed
or cannot be loaded (useful for suggested packages). It loads the package as
a side effect, because the package is likely to be used anyway.