An expression to be evaluated only if testvar is set to a
non-empty value.
testname
Optional name of the test. If given, and the test is skipped, will
print a message to that end, including the name of the test, and
instructions on how to enable it.
testvar
Environment variable name. If set to one of the yesvals,
expr is run. Otherwise, an optional message is printed.
yesvals
A character vector of strings considered affirmative values for
testvar.
lowercase
Whether to convert the value of testvar to lower case before
comparing it to yesvals.