Fix default build options

[rhornung67]

Change CMake option defaults so that only RAJA test build by default, and all other options (exercises, examples, etc.) are off by default.

Maybe also make build of tests off by default in Release mode, but on by default in other build modes?

NOTE: This will require a change to user guide to make it consistent with code.

[MrBurmark]

It would be confusing to have different things build in release vs debug modes, how about only having tests enabled by default.