Hide enable_testing() under PROJECT_IS_TOP_LEVEL condition.

[Bbllaaddee]

1. Bump cmake version to 3.21.
2. Create new FASTOR_ENABLE_TESTING option 3, Check if project is top level (not a dependency)
3. remove redundant enable_testing() from lower level CMakeFile.txt's

[Bbllaaddee]

This PR fixes #178