By default, it expands into a `static_assert` + `SUCCEED` pair, but
it can also be deferred to runtime by defining
`CATCH_CONFIG_RUNTIME_STATIC_REQUIRE`, which causes it to expand
into plain old `REQUIRE`.
Closes#1362Closes#1356
This means
* a new cmake option, `CATCH_BUILD_EXTRA_TESTS`, that conditionally
includes the ExtraTests subfolder
* building and running them on some of the Travis build images
* An example configuration test
In the future these should be extended to cover most of the
configuration options in Catch2, but this is a start.
