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.