isExpensiveExampleOk: Conditional checking for expensive examples.
Description
Queries environment variable
R_EXPENSIVE_EXAMPLE_OK. Returns TRUE iff
set exactly to TRUE. This allows conditional
checking of expensive examples in packages via R CMD
CHECK, so they are not run on CRAN, but at least on your
local computer. A better option than dont_run in
many cases, where such examples are not checked at all.