Added option to disable build of tests and samples.

Added FLATBUFFERS_BUILD_TESTS option which can be used to disable
the build of flatbuffers tests and samples.

Tested:
Verified tests and samples are no longer built on Linux when
FLATBUFFERS_BUILD_TESTS=OFF.

Change-Id: Ic23ab827849ba2c4481de9ca86adc1ab8e6b828c