Set cmake policy CMP0110 explicitly to avoid warnings

[springmeyer]

Using cmake 3.23.3 when building selene from source I see many repeated warnings like:

CMake Warning (dev) in test/CMakeLists.txt:
  Policy CMP0110 is not set: add_test() supports arbitrary characters in test
  names.  Run "cmake --help-policy CMP0110" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.

  The following name given to add_test() is invalid if CMP0110 is not set or
  set to OLD:

    `"selene_tests:Image view creation"´

This warning is for project developers.  Use -Wno-dev to suppress it.

The warning is repeated so many times its difficult to make sense of the other cmake output. So I thought it would be worth providing a PR fixing this issue. Happy to amend this change to do it differently, but for me this change allowed the warnings to be silenced.

/cc @kmhofmann