CMake: allow parent project to disable unit tests for streamvbyte

[emaxerrno]

While integrating into another cmake project via add_subdirectory() the target check might already be defined by a parent directory.

In addition, on a parent project when you run its unit tests you don't want child project unit tests to run

For example, this won't build the unit tests:

cmake -DSTREAMVBYTE_ENABLE_TESTS=OFF .. 

[lemire]

Does STREAMVBYTE_ENALBE_TESTS include a typo? ENABLE vs. ENALBE?

[emaxerrno]

@lemire Yeah! good catch. I fixed the typo.

Also squashed the commits, and cleaned up the history

[lemire]

Merging.